The recommendations for maintenance seem to be changing, perhaps because problems with the maintenance, itself, can create more issues than it solves. Defragmenting the hard disk was something that could be done for free and was often done frequently. Now, it's recommended only if fragmentation is so extensive that it seriously slows the computer. Cleaning up the registry used to be the province of paid programs but Microsoft now recommends not to do it at all. Updating drivers is often a service that is pushed to improve performance, but it is generally not a good idea to do it unless you are trying to solve a specific problem with a bad driver. Some of the maintenance programs routinely micro-manage memory or tweak things like network settings when you shouldn't have to do that more than once, if at all.
There is prevention, regular maintenance, and things to do only when you are having a problem. For example, manage the startup programs if the startup gets bloated and slow.
For prevention, good A/V software is obvious. WinPatrol, which WHS suggested, can save you a lot of headaches from stuff that would otherwise make changes to your system without your knowledge.
As far as maintenance, regular backups is probably the most important thing you can do.
I would focus on trying to make maintenance as unnecessary as possible. Follow good practices to avoid things mucking up your computer in the first place. Clean up specific issues when they affect performance but minimize excessive prophylactic maintenance.