Doug Knox's Exe File Association Fix;
Find that entry in the list, then follow the instructions for using it.
May I suggest that, after you have fixed this, you download, install, run, update, then perform a full scan with Malwarebytes' Antimalware. Anti-virus scanners no longer find all threats, and we need additional anti-malware help.
I was surfing with Firefox when AVG started to quarantine malicious files on my PC. After that, I tried to click on my desktop icons like Firefox or Mozilla, and a window would appear asking me to choose the program that I want to open the file with. It will unusually ask me to do that for some files (.exe files, video files), but not others (Internet Explorer, Photoshop PSD files) on my desktop.
I also tried to decompress a .rar file, but I get an error message that Program Files/winrar/WinRar.exe can't be found. Of course, the file is still on my PC, and I can open it.
Here is what's strange. I can right click on a desktop icon (Firefox) and select "start" and the program will open. But if I double-click the icon, Windows doesn't know how to open it.
I'm running Windows XP. I have no idea if this new problem is related to the last virus quarantine by AVG (paid version). I rebooted my computer, but the problem still exists.