Heyyo,
If you use that windows blinds or other programs that skin windows xp, I think those use some gfx, cause I can get some nasty framerate loss...
New OS are like inbred dogs, just cause they're newer versions of previous versions don't make them better, infact, it's more likely that it'll make them even worse.
I have my windows xp theme to barebones classic. I turned off a lot of the graphical features to get better framerates, and make the system faster. One of the worst ones for it is fading menus, that's one of the most cpu-intensive special effects that windows uses.
X (a gui for Linux OS's) seems to look just as nice, but run much more smoothly. Now, if only I could find a linux version of HL2 without having to buy it or illegally dl it... ah, Cedega, sounds great... but costs money... argh, I'll search later for a free alternative.