There is nothing like having your Anti Virus decide that a Windows system file is a virus as certain XP users of AVG8 found out on Tuesday.

It seems that when coupled with the Dutch, French, Italian, Portuguese, and Spanish language versions of Windows XP AVG 8 would tell the user that they had a Trojan and recommend removal of the offending bug.
Unfortunately this left most system completely unusable either in a reboot loop or unable to boot at all.

AVG did release a fix quickly to correct this

Read more over at CNET

On Tuesday an update for AVG 8 suggested that a Windows system file is a Trojan horse, and users who delete the file form the system could leave their Windows XP systems endlessly rebooting or unable to reboot at all. The problem only affects users of AVG 8 products running the Dutch, French, Italian, Portuguese, and Spanish language versions of Windows XP. AVG immediately sent out a corrected update to its customers, including those using the free editions of AVG.

A representative for AVG said, "AVG is actively working to remedy the problem some users are experiencing related to the most recent update to commercial and free versions of AVG 7.5 and AVG 8.0 in some languages. A number of users who installed the update mistakenly received a warning that the Windows system file user32.dll product version 5.1.2600.3099 was infected with a Trojan virus and were prompted to delete a file essential to the operation of Windows XP."