You didn't list your system specs. I know when I got my first dual core (Athlon 64 x2 4400+) on a win XP system, I had to edit my boot.ini file.http://support.microsoft.com/kb/833721
In your boot.ini file yo will see a line like this:
multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Microsoft Windows XP Professional" /fastdetect
Add the /usepmtimer to the end of the line.