You should also take a look at your Device manager and make sure you don't have any issues with any of your devices - something as simple as a faulty USB device could cause your system to lag.
Actually, the first thing I would do would be to disconnect
everything from the computer except the RAM, a keyboard, mouse and the primary HDD (and video card if you don't have onboard). Then boot up and see if it still lags. If so, maybe hook up your DVD drive and boot with a live CD like
Knoppix and see how your system runs under it - this will give us an idea of whether it is a hardware issue or software/OS issue.
If it still seems sluggish under Knoppix, you might start testing your RAM and your HDD with something like
UBCD. Could be faulty RAM or a HDD on it's way out. Also - what are the temps on your processor - an overheated CPU runs like sh!t. Check it with
SpeedFan.
Of course, a quality PSU could only help, never hurt.
Please post back with your results if you try my suggestions - we'll get that thing running right before the days over!
edit: (nm about the HDD - I missed where you'd tested it)