I've already mentioned how to enable the tools for overclocking your graphics card above. However, the concept and necessary details on how to safely overclock your PC are lengthy and would fill several guides. So I'm simply going to address overclocking from a troubleshooting point of view here.
Basically if your system is overclocked, you are running your machine beyond specifications. This can create several problems. First of all, individual components such as the CPU and graphics card (and even the hard drive) will build up additional heat for which their stock cooling may not be adequate. It can sometimes take quite a while for heat to build up in your case, and even with the case sides off, heat may still be building up and not dissipating fast enough from the immediate area around these components. This results most commonly in system crashes, lockups and even data corruption.
In fact, almost every lockup is due to heat or hardware failure of some sort. If you're experiencing any lockups then first of all try taking the case sides off, then try putting a normal desk fan facing into the case and see if that reduces the lockups or increase the length of time before you get one. If so then heat is the culprit. The only option you have is to buy better cooling for your PC, and in most cases, to also reduce the overclock.
Crashes to desktop are similarly often caused by overclocked components (particularly RAM and CPU) simply not coping with the overclock. Even if cooled properly, there's a limit to which a component can be overclocked and still remain stable. So wind back the overclock and see if it helps.
Finally, I personally value stability over speed. I mean there's not much point to having a fast system if you keep crashing every 30-60 minutes. You might impress people with your 3DMark2001 score, but generally gaming is frustrating on such machines. Most importantly, even if you crash in one game or app, or under rare circumstances, then you still have an unstable system.
In the case of Morrowind in particular, it really stresses system components. Even if all your other games run fine on your overclocked machine, Morrowind will probably crash it 9 times out of 10 simply because it's so demanding, whether through unoptimised code or whatever. Reduce your overclock dramatically and if you get a reduction in crashes then you know where the fault lies.
And on the topic of 3DMark2001, you can download it from here. It's a graphical benchmark which will give you a numerical result. The best way to use it is not so much to impress the opposite sex, but to see if the tweaks you're doing will increase your graphics performance (the 3DMark result increases) or decrease it. It's also a good stability test, because you can leave it to loop continuously for several hours as a simulation of intense graphical gaming and if you get a crash then your system is not stable enough.
Well that's it. I hope this guide's been of some use to you. If you have any additional tweaks or tips for Morrowind, or feedback (both positive and negative welcomed), please head over to the Gaming section of our forums!