As I haven't had the displeasure of downgrading from XP to Vista (lol), I don't know much about it. But, I can say that XP (bare) can run on about 20 processes. I'm running about 27 processes myself and that includes a vista skin. And I only have 768MB of RAM.
I have a book by: Steve Sinchak, called "Hacking Windows XP"
Wiley.com is listed on the back cover and they may have books for Vista.
The book describes the "system services" and which ones you can usually
turn off as well as a whole host of other tidbits that allow you to customize
your Windows software.
Also, Antivirus software can be highly resource intensive. Having 2 just doubles that.
in addition they don't usually play well together. Your best bet on that is to just pick
one and keep it updated.
And of course with windows I consider a fresh install as routine maintiance. So if it's
been a while since you've done it, try that. I've done it on an HP using the one supplied on the second hard drive partition. It's a lot easier to get rid of useless bloat-ware when you know it's a fresh install.
hope this helps.