Here's what I suggest...
1) Download ccleaner from download.com and run it to do the registry and file cleanup.
2) Uninstall every stupid little program you have on your system like google earth
3) Go through MSCONFIG to stop useless programs from starting on boot up. Here's how to use MSCONFIG if you're not familiar with it.
3.1 ) Click Start>Run
3.2 ) Type in "msconfig" and press OK.
3.3 ) Click on the "Services" tab and check "Hide All Microsoft Services"
3.4 ) Uncheck any items here that you are familiar with, such as itunes, google, quicktime, etc.
3.5 ) Click on the "Startup" tab and uncheck any items here that you are familiar with, such as itunes, google, quicktime, etc.
3.6 ) Click Apply, then OK or Close.
3.7 ) Save anything in an open program.
3.8 ) Click Restart
3.9 ) When your system boots up, check "Don't show this message or launch the System Configuration Utility when Windows starts" and then ok.
4) Since it's the huge topic everyone is talking about, I'll add my 2 cents in on cooling: make sure your fans are doing the proper air flow. The front one should pull air into the tower and the back ones should force it out. You definitely shouldn't have the fans at both ends pulling air in or pushing it out. And you might want to use twist ties to hold the powercables all to the sides so they don't obstruct air flow.
5) Get an Intel processor
I've been an AMD guy for years but I'll be switching to Intel when I build my next PC.
And finally... drum roll please... get rid of Windows Vista!!! That will solve 110% of your problems hahaha.
Hiren's boot disk is really good btw! If you don't like/trust memory testers, try removing some RAM stick by stick to see if you get improved or equal performance with less RAM. That would mean one stick is dead/dieing and your PC is too dumb to learn not to write to it.