CFP and hard disks

I'm running CFP behind a wireless router and Avast under XP Pro. I have two permanent hard disks and occasionally attach others via usb. The second permanent hard disk is used for personal data files and backup for the primary "systems" disk. Although I'm happy with CFP and the router firewall (famous last words!) at this time, I have lost data with this same combination in the past albeit on a different pc.

Is it possible to block the "internet" from ever being able to see, never mind access or infect the other disks - I'd rather use just the one disk for all internet work - programs, caches, temporary files, downloads etc - and physically cut and paste only the info I want to keep to other disks; is this possible, and how? (There are also other pc's in the wireless network which I would like to have access to these other disks)