I wouldn't recommend X64. Mainly because there aren't drivers avaliable for all hardware and there won't be any either. X64 is a dead OS because M$ have concentrated on Vista 64 instead.
I used to recommend AVG Free as a good virus software but I honestly can't do that anymore. I've had two infections these last few months that AVG didn't see. Both were found by Trendmicro and Kaspersky online scanners. One of those viruses messed my registry up and made it impossible to uninstall any software via Add/Remove program.
Don't use AVG, it's just not good enough.
One important part in having a good and responsive system is to have up to date drivers for all hardware. The default windows ones aren't always that good.
Using MSconfig to turn things off isn't a good idea either. Then it's better to go into Control Panel/Administration tools/Services and turn unnecessary things off. This should be done with caution though, turn the wrong service off and you have a system you can't use. There are some services that you can easily switch off though, they run and never do anything.
Defrag often. And don't use the default defragging software that comes with XP. It's worthless, it doesn't help at all. I use O&O Defrag and it makes wonders. System boots up a lot faster after a full defrag.
Don't install to many fonts. Windows scans through the fonts all the time. The worst software to use with many installed fonts is IE, it scans through for just about every webpage you visit..
Don't use IE, use Firefox.
Someone already said, don't install a lot of software just to check them out. Read up on them first and see if it's something you'll ever use. If not then don't. Even if you optimise the registry to many installed and uninstalled programs makes a mess of everything.
Well, that's a few tips from me.
