Here a few tools i use personaly,

http://www.lavasoftusa.com/software/adaware/ (Adware/Spyware removal)

http://www.download.com/Spybot-Searc...-10122137.html (Adware/spyware removal)

http://www.grisoft.com/ (Free antivirus software)

Once they have been installed make sure they are upto date with the latest updates and if the owner of the pc you purchased did leave any thing undesirable behined it will more then likely be picked up by these programs and be removed.