Run Microsoft Security Essentials: If you're using Windows, you'll probably want some form of antivirus software running in the background. There's no need to pay for antivirus, as Microsoft Security Essentials is an integral part of the latest Windows iteration, Windows 8. Security Essentials has been integrated with Windows Defender and Windows Firewall. Antivirus or no antivirus, firewall or no firewall, the best defense against malware is safe browsing. So be responsible while browsing.
Update Windows: First thing you want to do is keep the OS updated. Don't neglect Windows Update. Install updates whenever made available and follow the instructions. This will keep your computer safe and stable, and really takes no effort. Updates not only happen for Windows but for other installed Microsoft products as well.
Clean Temporary Files: Windows can leave a lot of clutter and temporary files lying around your hard drive, and it's a good idea to clean these up regularly. Try using CCleaner. Though I've not tried the application, I've heard from my friends that it does clear the clutter. Also heard that you can schedule it when to do the cleaning.
Back Up Your Hard Drive: Whether it's the simple Windows Backup or an automated, offsite backup, manual, let it be anything. Backups are essential tools for any PC user. It isn't just maintenance, but it can get you out of tight corners. Find more about backups here.
That's a short list of Windows maintenance tools. These are pretty much the essentials. They should keep your computer running in pristine condition for a while.