Got called out to a pal's PC this week, which was in a shocking state with various trojans and viruses infecting it.
The problems stemmed from the fact that my pal had deleted his security software to make room on the hard disk for some videos and music he was copying^H^H^H^H^H^H^H - sorry, backing up.
One of the Trojans appears to have corrupted the registry of the PC to such an extent that Windows Genuine Advantage thinks the version of Windows on his machine is pirated.
It's not, but perhaps worse, a number of other applications, including AVG Anti-Virus are also blocking updates until the copy of Windows is made legitimate.
I think a complete re-install of the operating system is called for, as patching around the problem seems to be impossible.
It could be that AVG's program is corrupted, or it could be that Grisoft (the firm behind AVG) is simply using the Windows licence flag as a means of checking whether the PC is legit or not.
If it is the latter, then this is a serious development...