A general explanation of IRQs in Windows XP by Microsoft can be found: Here.
Interrupt Requests (IRQs) are the way in which all of your major system devices get the CPU's attention for instructions/interaction as often as necessary. There are 16 IRQs (0 - 15) in most PCs, although some newer PCs have 23 or more IRQs. Each IRQ has a priority assigned to it as to which gets the CPU's attention first if several are competing at once. It is easy to imagine that while Windows is designed to allow several devices to share an IRQ, you may have problems or reduced performance if two or more major devices (such as sound card and graphics card) share an IRQ.
Under Windows XP, as the MS article above explains, ACPI (Advanced Configuration and Power Interface)-compliant systems - which is most modern PCs - will have their IRQs automatically allocated by Windows. Even if you manually assign them in the BIOS, Windows XP will reassign them again. So let's examine the IRQs more closely and see what exactly we can do under these conditions to prevent/resolve IRQ sharing and conflicts.
The default assignments for 16 IRQs under Windows XP are:
The IRQs marked RESERVED cannot be changed by the user, and are the same on most systems. These IRQs are pretty much taken, so don't even think about trying to get another device on them.
The IRQs marked AVAILABLE may already be taken up by specific devices (where indicated in the brackets) if these devices are present and not disabled. For example, most people have 2 IDE Channels enabled, the Primary IDE Channel for their hard drive and first CD-ROM, and the Secondary IDE Channel for another hard drive, and/or for CD/DVD/CDRW Drives. If on the other hand you don't use the Secondary IDE Channel at all, and disable it in the BIOS (see below), then you can release IRQ 15 for another device. The same with the Serial Ports and Parallel Port for example.
If you want to reallocate your IRQs because several major devices are sharing, first do your maths. Of the total number of IRQs, at least 8 are reserved and cannot be taken. That leaves you with 8 IRQs free at most if you have 16 IRQs, or 15 or more if you have 23+ IRQs. Typically most people have both IDE channels enabled, and at least another IRQ taken up by a Mouse (USB, Serial or Parallel), so that's another 2 IRQs taken up. So for starters if you have more PCI, AGP or COM devices than potentially free IRQs, you'll get IRQ sharing no matter what you do.
If you still want to proceed with trying to change the IRQ arrangements, there are several methods I know of which will work with varying degrees of success. Remember, under Windows XP there's pretty much no other way.
Update Motherboard Drivers and BIOS
The first step is to update your motherboard drivers and if possible, install the latest BIOS for your motherboard. See my System Optimization Guide for details on where to get them and how to install them. Note that this step is unlikely to have any great impact, but it may help in the reallocation of IRQs as part of the next few steps. Also, updating your BIOS is something you should do anyway, and who knows maybe you'll be lucky and a new BIOS might end up giving you more than 16 IRQs.
Disable Unused Devices
1. Reboot your PC and go into your BIOS.
2. Disable any devices which you do not use. Obviously you should make sure nothing is connected to these devices as they will stop functioning if the device is disabled (e.g. a Printer on the Parallel Port). The logical choices, given the IRQ allocation table shown above include:
Serial Port1 (COM1)
Serial Port2 (COM2)
Parallel Port (LPT1)
Secondary IDE Channel
3. Once you've disabled as many of these unused devices as possible, find an option in your BIOS like 'Reset Configuration Data' (or similar), and activate it to reallocate the freed up resources on reboot.
4. If you reboot into Windows and that hasn't worked in changing your IRQs, which is quite likely, move on to the next step.
Physically Move Devices
1. Physically shift a device sharing an IRQ with another major device if possible. For example, if your sound card is sharing an IRQ with another major device, turn off your PC.
2. Shift the sound card from one PCI slot to another free one.
3. Reset your configuration in the BIOS and reboot your PC.
Once again this is unlikely to work completely because of the way Windows XP may simply reallocate the same IRQs to your devices on ACPI-compliant systems. If it does fail, move on to the ultimate step below.
If all else fails and you're having serious problems with devices which share IRQs, then the relatively foolproof way is to follow all the steps above in sequence, then quite simply reformat your hard drive and reinstall Windows (See Installation Issues section). This has the highest chance of reallocating the newly freed-up IRQs, though if you're unlucky and you have a large number of devices Windows may still force 2 or more of them to share. As long as they're not major devices though, things should be fine.
The reformat/reinstall method is the only guaranteed way of getting the IRQs to change. By freeing up several IRQs (especially by disabling COM/LPT resources), and given you have 6 or less additional devices on your system, you have a very good chance of winding up with no, or minimal, shared IRQs.
Please note that I don't recommend disabling ACPI. This is a common solution to IRQ sharing which some people suggest, but it can cause a raft of problems and disables functionality like being able to shut your PC down through Windows (you'll have to use the power off button) to name but one. Strange system behaviour may also result, perhaps worse than any problems you had with IRQ sharing in the first place. You also cannot reenable ACPI without reinstalling Windows again anyway, so it's just not worth trying.
Changing Drive Letters
Tool: Computer Management
If you have several Hard Drives and CD/DVD/CDRW Drives, you can use the Computer Management tool to change the drive letters allocated to them by default (e.g. C:\, G:\, etc.). To do so, follow these steps:
1. Open Computer Management by going to Start>Run and typing "Compmgmt.msc" without quotes.
2. Under the Storage area, click on Disk Management. You should see all your drives listed in the right pane. Use the scroll bar to look through them all.
3. Right-click on the drive name in the right pane whose path you wish to change and select 'Change Drive Letter and Paths'.
4. Click the Change button and from the drop down list on the right, select a new drive letter. Click OK and click Yes on the warning tab - as long as you realise any shortcuts referencing a program on the changed drive may be incorrect now.
Note that if you want to swap two drive paths, choose an untaken path for the first drive (e.g. Z:\), reassign the second one to the new path you want for it, then go back to the first drive and reassign it to its new path - you cannot set two or more drives to the same path at the same time.
How much to upgrade to x64 Edition?