Pentium 4 3.6GHz, Singlecore
1 Gig of DDR (1)
MASSIVELY (70-90%!) fragmenbed ATA133-Disk
A stinkin old ATI 9600
Windows XP Pro SP2 fully patched
AVG antivirus
UFO:AI RC6 out of the box-install.
No drop in gamespeed whatsoever.
I can defrag the disk while playing: No slowdown...
I can crunch cancer-calculations (100% CPU consumption on lowest priority): No slowdown...
I can massively copy stuff from one HD to another or burn DVD's: No slowdown...
Maybe it is the antivir... If you have Norton: Norton is known for his CPU-hunger and slowdown-ability.