since you said you don't know anything about it then I would run a belarc to get all the info about the pc and a SIW, plus you will want to run a secunia scan to see what needs updating on it and do those asap. Belarc Advisor SIW those are good to print so that you have all the info handy should you need it. Secunia Online Software Inspector (OSI) I forgot to mention adding spywareblaster to your security list, just install, update then once a week open it hit update then click the enable all protection so the shield turns green. SpywareBlaster be sure you get the free version when you go to the download link just continue with the one you started with do not move to the trial of the pay version.

http://www.microsoft.com/windows/products/winfamily/defender/support.mspx And, Windows Vista is an operating system made by Microsoft which contains a built in malware protection tool called Windows Defender, which can also be downloaded and run as a stand alone service for Windows XP. Windows Defender is a part of the Vista operating system, so it cannot be removed without risking total failure of the operating system. Hence, for Vista users, Defender should be disabled if it's services are not needed. In Windows XP, the stand alone install of Defender can be uninstalled normally. http://www.ehow.com/how_4881875_uninstall-windows-defender-manually.html

That's a paid program if it's up to date and running ok I would keep it. You should uninstall Microsoft Security Essentials Lavasoft Ad-Aware version 8.1.4 Features  Complete Malware Protection, Including Anti-Virus and Anti-Spyware  Ad-Watch Live! Expert Real-time Protection  NEW! Genotype Detection Technology  NEW! Behavior-Based Heuristics Detection  NEW! Expert Level Rootkit Removal System  NEW! The Neutralizer Malware Removal Tool  NEW! Do Not Disturb Mode Minimal Strain on System Resources  NEW! Download Guard for Internet Explorer  Toolbox Including Process Watch, AutoStart Manager, Hosts File Editor, and ThreatWork  Customizable Profile Scans  Network Drive Scanning  Automatic Pulse Updates & Free Technical Support Running more the 1 Anti virus program can cause slow downs, crashes. See link for uninstalling Microsoft Security Essentials. Here is a link that might be useful: Uninstall_Microsoft_Security_Essentials

The Microsoft link above is the best I can do for now, looks like a nightmare to me with Netframe work involved. There's a tool you can run that may or may not help, Windows update readiness tool. Dial-a fix (XP Only) may also help it's a tool that re-registers all the windows up-Date dlls. Lots of others have had issue with it too!