Really for laggy Windows doing what ^^^^ he said gets most of your processes down. Hit cntrl+alt+delete and go to processes tab. If you have anything over 30 and you dont have 2gig of ram it will be laggy. I myself, have 19 on internet drive, 13 on games drive.
You can do his registry thing (for services) or what i said to take off a bunch of services which in turn decreases your processes. Also, i recomend going to www.hijackthis.de and DLing their little program to get rid of a bunch of start items and possible threats in your bootup. (usualy gets rid of malware and spyware).
My other trick is plain and simple, disable themes and extra nonsense from windows.
Right click My Computer/ properties/ advanced/ performance settings tab/ disable all the nice little stuff Windows gives you (visual effects).
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For people with LOW RAM or just dont care for that stuff.
My dad has 42 processes and a gig of ram and its laggy. When i got rid of all the extra junk taking up RAM it was fine again. Most of your Windows laggyness (or slowness) comes from extra crap running in the background. If he means lag by video lag then its video card. I doubt thats the case.