Okay. I'm a Vista user, so I know how much it sucks (performance-wise). For starters, the comp must've came with pre-installed software. Make a backup of the factory settings and get rid of the useless junk. For me, it's HP's darn useless software, which starts with windows and drains the resources. According to that program, a small 'update of HP's quick-launch toolbar' is 'critical', and I can't ignore or hide that. It keeps prompting me to update. I just disabled the whole thing. Likewise, there will be many useless stuff. Remove everything. Next, check the programs/processes running in the background, like tray applications. Bring them down to a minimum. Then, the anti-virus. Which anti-virus did you get? If you got Norton, I recommend you get rid of it asap, it's the worst program I've seen, it practically drains the system resources, besides making you lose your bearings in the interface.
Now, during gameplay, disable programs like anti-virus, and disable Aero theme and switch to Vista Basic theme if it makes a difference. This should increase the performance by a bit.
Vista has a lots of stuff going on in the background, MANY windows processes which we might not need at all. I don't know the details, but search the net for optimizing Vista. While you're at it, DO NOT follow anything that requires you to install programs. And I recommend you don't mess with anything to do with registry. Disabling some Windows services and th like should be fine.