Bizarre problem. Could be virus-related?

[Mindee]

Hey, I have the strangest problem happening on my computer.  Actually it's my husband's computer, but anyway, he said he hasn't done anything different with it, but somehow everything has turned into a pdf file.  I mean every icon on his desktop, every program file on his hard drive---is now a pdf---that cannot be opened.  His email (outlook) is not affected.  And by some miracle I was able to open this one window, although sometimes it won't let me open everything.

Any ideas on how to solve this?  He left for work and asked me to look into it.

[Mindee]

ugh, and i can't open any of the security features and i couldn't open an anti-virus program after i downloaded it.  i can't even open the backup and restore center.

[kuszmania9999]

http://www.computerhope.com/forum/index.php/topic,46313.0.html

We still not sure about this but follow the instruction above.

Well, from what i understand, even if you have ANY FILE it automarically showed as a .pdf, right?
Can you right click and choose to open it with what?
For example, a .mp3 open it with media player?

[Mindee]

Actually, all his documents seem to be fine (not pdfs), but everything on his desktop and his programs themselves have the pdf icon. 

If i try to open the programs from the start menu, i get an error message that says "This file does not have a program associated with it for performing this action.  Create an association in the Set Associations control panel." And if i try to open something off the desktop, it says "Adobe Reader cannot open such and such file because it is either not a supported file type or the file has been damaged".

If I open, for instance, an Office document from his document folder, it opens fine (and the icons in his document folder are normal).

I was able to run Windows Defender (nothing found there).  However, I cannot install an anti-virus application (I get the Adobe error message above). 

I did look at his program list and he has nothing on the malaware list.  He really has very little on this computer, actually.

What do I do about the anti-virus?