I perform 2 full system images once a month on 2 seperate harddisks. It's about 150 Gb. (tripleboot XP, Vista en Windows 7 Ultimate).
Each time when I make an important change (Windows updates, new programs installs....etc.....etc) I write it down in a word document. So when an important change causes malfunction of my system and when System Restore solves nothing I know exactly what to change after restoring an image.
In between I backup my important files and folders manualy.