cleanup spyware and virus's . probably bogged down with crap.
If the pc performance decreased dra matically, why Installed software running all the time in the system tray if its on the bottom right by the clock and you dont use it close it, remove programs from startup you dont use , when is the last time you defragged the drive.
if you are already ontop of all this stuff. I secon the memory upgrade. you didnt say anything specific so it may or may not be necesary
