External USB hard drives are becoming really cheap now, as other people have said.

For backup I just bought a Western Digital My Book which has two 500Gb drives in it - cost about £150. It's a black box about the size of a (thick) paperback, with a mains power supply and a USB2 connection to your PC. It comes with backup software, and you can use it either in default RAID 1 configuration (500gb available space, mirrored disk) or RAID 0 (no mirroring, 1Tb available). In RAID 1 configuration, if a drive ever fails, the box just carries on and you can get a new user-installable drive from WD (free in warranty period)

I stuck the software CD in the drive and it more or less configured itself - all my photos, docs etc are now backed up and the software runs in the background backing up data as necessary. No doubt there are cheaper ways of securing data but I must say this is a pretty painless one (no, WD aren't my employer!)

Yes I know I should take DVD backups and use a "cloud" backup facility as well but I'm not very well organised....