Disappearance of application exe files in XP

I am unable to install programs under my XP-SP2 although I can run quite a number of them.

It all started when I noticed that my Sygate and Avast icons in the taskbar had disappeared. The corresponding ashAvast.exe and Smc.exe, the executable modules, were no longer present in their respective directories. I immediately cut off the link to the Web.

When I reinstalled these programs, the executable modules did not reappear.

The same thing happened with Spybot and Advanced Uninstaller Pro in that their exe modules disappeared.

Furthermore, whenever I open Windows Explorer, Word or Excel 2003, Windows Installer starts up and asks for Adobe Acrobat 7.0. I ran msicuu2 version 5.1 to no avail.

I used the Add/Remove function in the Control Panel to get rid of Adobe. It asked me for the Install CD. When I inserted it, Windows Installer told me it could not find it.

I can access the Web.

Would there be a virus? Should I try an online virus scan despite the lack of protection? Any particular site you would recommend?