I was creating a virtual this evening in prep for moving boxes around on my network and I did something rather magical.  Long story cut REALLY short, I associated notepad with .exe file extensions ... which seems like no big deal, UNTIL you try to undo it.  Now if you let this simmer for a minute ... you have no regedit, no explorer, no command prompt, NOTHING with a .exe will run in anything except notepad.  They're done, gone, bye bye ... ok not GONE but ... just WOW.  What's worse, I'm not the only person who has done this. 

As for the fix, make yourself a registry key with the following data (in yep, NOTEPAD!), save it as whatever.reg and double click and merge it (worked on 2k3 and looks like it could work on a lot of other versions, vista)

[HKEY_CLASSES_ROOT\.exe]
@="exefile"
"Content Type"="application/x-msdownload"

[HKEY_CLASSES_ROOT\.exe\PersistentHandler]
@="{098f2470-bae0-11cd-b579-08002b30bfeb}"

Anyway, just thought I would share.  Oh and restart after you un-do the snafu and expect it to be ...um, angry.  Yes, I'm saying you might have to/want to just ... reinstall.