ok first off, is the computer slow but stable or slow and unstable?
by stable i mean:
If you attempt to launch a program, the PC eventually does it (even if it doesn't perform well once launched)
The system does NOT lock up (will freeze at times but always seems to catch up eventually)
No BSODs/Unannounced reboots
If that is the case, then the problem may not be hardware related but something on the system is bogging it down
open task manager (Ctrl+Shift+Esc)
Click the processes tab
Check Show processes from all users at the bottom
You should see 4 different users listed, NETWORK SERVICE, SYSTEM, LOCAL SERVICE, and your username
Sort the columns by CPU and look to see what is highest on the list. If all is well, then the 'System Idle process' should be the highest, if not, then if the process that is higher than System Idle is running as your username, you should try ending process on it and see what happens
Sort the columns by Mem usage and look to see what is highest on the list. If anything is using WAAAY more memory than anything else and is running as your username, you should try ending process on it and see what happens
If nothing there looks out of place, post a log from
HiJackThis! and well look it over to see it you have any unwanted programs on the system.
If the system is NOT stable then i would run
http://www.memtest86.com/ youll have to download the ISO and burn it to cd (prolly on another PC) that will test your RAM to see if it is the problem