first thing to do. Download the reg file from the EXE File Association Fix for Windows XP section. This should bring back the runniong of exe files. if that doesn't work than read on...
you said you know you dont have a virus because you've formated over and over. You aren't perhaps backing up files before the format and restoring them (and, with that probably restoring the infected files), are you? Another thing is to NOT install Nortons, but run a trial at first before installing Nortons. Anti virus software can cause these problems, too. Install it later, but for now dont install it just for testing purposes, as well as any other intrusive (but constructive) program.
Anyhow, mostly make sure to NOT restore backed up files right away and see how it runs. Like stated above, maybe your also restoring a infected file.
Actually, if your able to open a browser goto tendmicro housecall and run the free online virus scanner and have it fix it. It's better than many other virus prot programs and is upto date since it is online. It doesn't take the place of Nortons (because it is just a scan/fix and wont PREVENT anything in the future) but will find most anything.