Windows Updates
Windows updates are automatically downloaded by the computer. Check for this icon in your taskbar, click on it to start the installation. It may take a few minutes for the icon to come up after turning on the computer. If it doesn't show up then your PC should be already up to date.
It's also recommended to install the updates at end of the day to save time. You may pick "Install updates and shut down." when logging off.

Java Updates

To make sure your Java is up-to-date:
Start Menu -> Settings -> Control Panel -> Java -> Update tab -> Click on Update now
When running the Java updates, be sure to uncheck any Internet Explorer add-ons.

Forefront Updates

Forefront will install updates and definitions automatically.

Disk Cleanup (recommended monthly)

Start Menu -> Programs -> Accessories -> System Tools -> Disk Cleanup
Check all boxes except for Compress Old Files and Office Setup Files

Clear Internet Explorer Cache (recommended bi-weekly)

Open IE, go to Tools -> Internet Properties. Under "Browsing history", click on delete and check all the boxes.
If there are unwanted toolbars on your IE, you can do a factory reset under the Advanced tab in Internet Properties.

Hard drive scan / Disk defragmentation (recommended yearly)

Open up "My Computer", right click on C: drive and click on Properties. Click on Tools tab. From here you can schedule a disk scan on restart by click "Check Now..." or run Defragmentation by clicking "Defragment Now....".
Either process will take a very long time.

Clear Java cache (recommended whenever you're having trouble running Java applets)

Start Menu -> Settings -> Control Panel -> Java -> General tab -> Settings... -> Click on Delete Files... -> Check all boxes and OK -> Click on Restore Defaults