Depends how much data you got to clean-up to get back the space and delete temp files that can be tracked by spyware or adware or websites, or any unwanted files that caused slow in using the system.
You can use CCleaner (freeware from http://www.ccleaner.com/) or use online cleaner provided by Microsoft http://safety.live.com/ (Note: If you will use the Live Safety by Microsoft, make sure that System Restore is enable and running)
I, for one, run clean-up in daily basis.
I have the following anti-spyware and anti-malware software on my system:
3. Spybot Search & Destroy
4. Avast Antivirus
and I have ZoneAlarm as my firewall.
How often should I cleanup my system, is once a week enought?
Thanks in advance.