Surely it's the job of your anti-virus software to keep your system clean!

As a corrupt or otherwise useless backup is pretty much catastrophic, the issue of testing backups is critical. Performing a controlled restore is usually the only sure fire way of testing a backup, but how do you do that? You certainly don't want to do that on the PC you've just backed up, as a corrupt backup file than trash your PC (and you've just proved you don't have a backup). The only solution is to backup to a spare PC, which must be identical.

I prefer to copy files to a write once optical disk. You can then take this disk to any PC and try reading the files off it. I've just bought a Blu-ray drive, so I can get 50 GB on a disk!