Gary
I'm talking more about the clean-up/tune-up utilities not so much the anti-virus tools.
The secret of a stable system is to keep things as vanilla and avoid installing anything you don't really need. There's very little need to take a machine back to the shop just to be cleaned up, just takes a bit of discipline
Some ideas. Most obvious but some others may benefit:
1) Never save data to the system drive (C: drive) of your machine. That drive is for the machine to use - not you. If you have a second hard drive (preferrable) or a single drive with two partitions for system and data(less preferrable) your data should go on this second drive. Then if the system drive goes pear your data is at less risk.
2) Consider saving your data to a space independant of your system. For a home user this could be an external drive or even one of the better cloud storage services (Amazon, SugarSync, SpiderOak). Only consider an external drive from a drive manafacturer that's willing to put their name on the outside (Western Digital, Seagate).
3) Use a snapshot tool (TrueImage) to snapshot the machine in an out-of-box state (operating system with no installed applications). Once your main apps are installed take a second snapshot. Then snapshot your machine *before* installing any additional apps or drivers. You can then return to a previous working state if need be in the future. Windows System Restore is ok but something like TrueImage is much better. Use a second external hard drive (the small bus powered portable drives are ok for this) dedicated to these snapshots. Lock this drive away in a safe place away from your system.
4) Avoid the temptation of installing "trial" software. Ask yourself if you really need to install that little bit of software you just stumbled across and if you do, wait a day or two to ensure that you really do. Then ask yourself again
5) Keep your anti-virus software up-to-date. I favour Microsoft's free Security Essential which is as good as any I've used and from my experience has no noticable effect on system performance. Avoid the Norton's and McAfee rubbish.
6) Keep Flash and Java up-to-date, and use Windows Update to alert you of system updates.
The above should keep you reasonably safe and secure and will make any major system failures far less problematic.