S.C.
System for games -> Windows or cheap PC/Mac and XBox/PS2 or Gamecube
System resistant to viruses, spyware and hacking -> Just about anything but Windows or Windows loaded up with anti-virus, firewall and anti-spyware programs (which will hurt performance too)
Not really the best choices to have to make, but that's what we have.
There are 64 bit versions of Linux available that can fully utilise the Athlon 64 processors, and there will be a version of WindowsXP out soon too. Otherwise you can just run the current 32 bit versions of XP.
goates