Good advice to clean the c**p, no point in scanning old internet cache, temp, files etc.
Best to have a schedule for regular maintenance then nothing is missed and finally when every thing is done run avast before your back-up:
Sunday
Battery Charge - wireless mouse, etc.
Update & Run security programs - Adaware - Spybot S&D - Spyware Blaster, etc.
Temp Cleaner Utility of choice to:
Clear Cookies - Clear Browser Cache - C:\Documents & Settings\User\Local Settings\Temp
Defragment as required
Run a registry cleaner - Fortnightly - Caution, this can seriously screw you system, don't get carried away.
Run VRDB - Fortnightly, I do mine manually rather than with screen saver or
Run avast! AV
Run Drive Image Backup
I also back-up all volatile date files (docs, favorites, address book and email, etc.) every day. So if I do experience a serious crash at worst I lose 7 days worth of program installed (but should be able to re-install them from daily backups) and 1 days worth of volatile data.