Windows Updates
Along with good, up to date anti-virus and anti-spyware (AV/AS) software, keeping Windows up to date is the best way to keep your computer in tip top shape. Automatic updates are turned on by default in all Windows versions.
You can customize the automatic update settings by clicking on Start, Control Panel, Automatic Updates (Windows Update, Change Settings in Vista/7). Options include if/when to check for new updates and whether or not updates should be automatically downloaded and applied or simply notify you that they are available (a good idea if you use dial-up internet). You can even choose to turn off automatic updates all together but it is not recommended (and Windows will continually remind you of this).