Here are the basics of what I often use/install on systems:
AVG Free-- Anti virus scanner
Adaware SE Personal-- Anti spyware scanner
Microsoft Antispyware-- Anti spyware scanner. Windows XP Home and Professional only.
Spybot Search & Destroy-- Anti spyware scanner
ZoneAlarm Free-- Free firewall - more user friendly
Sygate Personal-- Free firewall - more configuration options
And the applications I use to edit the registry are:
Ccleaner
Registrar Lite.
-- Perfect for manually editing the registry.
EasyCleaner
-- Very swift program.
The only memory diagnostic application I have ever used, is the one provided by Microsoft and recommended by Merlin. It seemed to work well.
I also recently started using WinPatrol, after it being recommended by Fed. It intercepts programs that wish to add theirselves to Windows start up. Quite useful, but more useful if a computer is allready infected.
Other than that, I do not use any other applications that I can think of at the moment. Most tools that I use are part of Windows. Bound to be more compatible.