Here is a short list of things to do that will keep you and your PC happy for a long time (with the first assumption that you are keeping the OS and programs updated with Microsoft Update):

Do these weekly:

• Clear your cache's (run CCleaner, or "disk cleanup")
• Defragment your drives
• Run a virus scan
• Scan for Spyware/Malware
• Incremental Backup (or do this one more often)
• Backup your emails

Do these things once a month or so:

• First, Clean the cache, Defrag, Scan for viruses and spyware,
• Then, do a full backup
• Now you're ready for some heavy lifting: Remove unused programs in the control panel
• update system drivers
• run scan disk
• clean your fans (vacuum out the box)

Then, once a year or so:

• update the BIOS
• re-install windows as needed to get things REALLY clean (or just re-image with a program like Norton Ghost or my favorite, CloneZilla)