If you are worried about viruses, you should backup your data. Your data is a valuable thing indeed, and viruses is only one of the many ways it could be damaged. In the last year, I've had two friends lose their data when their external hard drive was stolen along with their computer, and my brother lost megabytes of notes when he accidentally overwrote an important folder.
I tried dozens of backup software before making this post. There is so much awfulness out there, for such a basic function, it's unbelievable. Backup, it's simple, how hard I can it be?
Here are my recommendations. Normal people will want Mozy. It is click-install-and-forget. It does off-site backups, so you are protected against the house burning down. It might not be open-source software, but it's free up to 2 GB. Moreover, at $60 a year for unlimited space, Mozy is the cheapest disc space you can rent.
If you have objections against safeguarding your data on somebody else's computer, try SyncBack. After some fiddling and tweaking it will do an OK job. Necessary trick #1, configure four backup schedules : daily, weekly, twice weekly, and monthly, each with a different destination directory. This way your backup contains four different versions, and you can revert to a good copy of your files ever becomes corrupted. Trick #2, buy two external hard drives and leave the second one at your mom's. Then, each time you go visit her, exchange the two drives. Your off-site enclosure becomes the bedroom one, and your bedroom one becomes the off-site one. Just ensure you switch them with some regularity, otherwise you are vulnerable to thieves and firemen.
Finally, geeks will want rdiff-backup. It is the only open source software I know that does incremental backups with byte-level diffs for both transport and storage.
Update, Nov 09, 2008
A full bitwise system copy done in hardware. Best solution, but somewhat expensive. If you are serious about off-site backups, you will need two.
A really cool combo consisting of online backup space + collaboration sharing + web sharing. Slightly more expensive than http://mozy.com :
Online backup priced at 15 cents per gigabyte. Cheaper than http://mozy.com if you need less than 33 gigabytes. Three-way portable Win, Mac, and Linux.