Download windows defender and run it. It is free. Get the latest patches.
I discovered just yesterday what I believe was a conflict with Defender and AVG (paid full security version). I uninstalled Defender. I kept receiving Msiinstaller warning message in my System event viewer. During the uninstall I was required to close Outlook. I presume both were monitoring my email and that was the conflict. So far so good—clean event viewer.
Best page to do research with MicroSoft Valued People (MVP) expertise.
http://www.microsoft.com/windowsxp/expertzone/newsgroups/reader.mspx?dg=microsoft.public.windowsxp.general