EXE files not opening

Hello,
I recently re-installed WinXP on my Dell laptop as I had been having issues with it. After this, I tried installign AVG Free, but it wouldn't install - a Google search revealed that other people had been having the same problems.
So I tried avast! Free antivirus, but it kept removing files that were perfectly safe. I didn't want my files to be disappearig, so I uninstalled it. Unfortunately the files that it had moved to the vault did not reappear on my computer, but luckily I had them backed up. The major issue that I am experiencing is that since uninstalling Avast, .exe files won't open on my computer - a box comes up for me to choose what program to open the file with.
Can anyone offer any help? I'm not sure if it is a virus or what the problem is... the other possibility is that it has something to do with AVG - I tried installing it again and although it now appears in my programs, I can't open it and it doesn't seem to be working.

Thanks