Download and install the following programs, all can be found very easily on a google search and dont take up very much space at all.
AVG: Scans computer for viruses. Updated frequently.
Spybot Search & Destroy: Scans computer for any spyware and malware, basically anything that slows down your computer, also is rather fast scan. Updated frequently.
Ad-Aware (Lavasoft program): Does a very deep search through your computer finding anything that is wrong. Updated very frequently.
All these are the programs i use and im yet to have a problem with a virus or spyware. These have always found my problem. Also if your computer is slow maybe you have to many things running on startup, in that case i suggest the program 'Codestuff starter' its very simple to use and its a very small download.
Hope that helps a little
