Best way to clean a PC

[Eminem]

By clean I don't mean physical cleaning. I mean the HD. My PC suddenly slowed down tremendously and I have been taking lots of steps to sort it out...I performed the following tasks:

Registry Cleaner
Spyware Scan
Virus Scan
Cleared History + Cookies

Will do:

Defrag it.
Clear out temp files.

I also used Msconfig to sort out the startup programs...Any other tips will be helpful.

Thanks.