The only problem with something like that is I heard running a program through a mounted drive like that uses so much more RAM. (Is that true, in your experience?)
I use Norton Client Security, it's fast, reliable, has lots of user control, doesn't use much systems RAM like over Norton Products, and has a 64-bit compatible antivirus, plus no subscription fees.
Anti-Virus: NOD32 and Symantec Client Security.
Anti-Spyware/Adware: Spybot: Search & Destroy, Microsoft Anti-Spyware, Ad-Aware SE Personal Edition, and Webroot Spysweeper.
Firewall: ZoneAlarm Pro
The best thing about all these software? I got them for free.