do you have scripts running infinite loops?
how many scripts do you have running?
what is the speed of your processor, quantity of video RAM, and and quantity of RAM?
if you meet any of the following conditions, try simplifing the situation when it comes to instructions.
there is also a benchmark command, which I never needed to use, which might help tackle lag, check the official command reference for more info.
try deleting all tempory internet files, cookies, and temp files (C:\WINDOWS\Temp), remove programs you don't use, delete invalid shortcuts and programs that cause errors or system instability. Defrag you C: drive after you do this.
after that end all applications but explorer.exe and task manager (if windows XP). Also what's in that startup menu?