Simple PC Maintenance
Disc Clean-Up: Deletes Temporary files and other clutter with the option of compressing.
Registry Clean: Run registry scan to remove missing or invalid entries.
Defragment: Reorganises the stored data for efficiency.
Fortunately, Microsoft now has an automatic tool that does all this for you.
Go here and click 'Clean Up', then follow the instructions.
Keep Your Hard Drive Clean
Antivirus Program: AVG Free Edition also has a link scanner to warn of 'poison' websites.
Download AVG Free.
Anti-Malware: LavaSoft AdAware scans your system for cookies and malicious software [malware, greyware].
Download AdAware.
Registry Deep Clean: Registry Mechanic [not free] deep scans your computer for invalid or missing entries.
Download Registry Mechanic.