The following are what I personally use and recommend for others to use to protect and clean up their computers. They both work on Windows XP & Vista. Please view their sites for more information and compatibility with your computer before downloading.

AVG

Anti-virus & anti-spyware ( http://free.avg.com )

AVG is a completely free program that offers real-time protection, regular updates, and scheduled scans.

C Cleaner

System Optimization, Privacy and Cleaning Tool ( http://www.ccleaner.com )

C Cleaner removes unused files from your system – allowing Windows to run faster and freeing up valuable hard disk space. It also cleans traces of your online activities such as your Internet history. Additionally it contains a fully featured registry cleaner.