There seems to be a problem with the associations, probably something like:
A virus has associated executable extensions to itself; then, the virus has been deleted but the registry entries were not fixed. Now, every time you start an executable, the system is trying to start the virus executable that is still registered to be used - and fails.
Can you start any other (non-exe) file - such as .com, .bat, ...?
If only .exe association has been change, that it is possible to repair the problem by renaming the regedit.exe to regedit.com and fixing the corrensponding registry entry, i.e. setting the default value of
HKEY_CLASSES_ROOT\exefile\shell\open\command
to
"%1" %*
However, if it was Swen, then it has associated even on .com (and others, such as .bat, .scr, ...) extensions, so you will not be able to run anything.
If you have WinNT/2000/XP, you can try renaming the regedit.exe to regedit.cmd and run it. For Win9x, heavy DOS hacks would be required...
Btw, avast can remove Swen using its intergrated Virus Cleaner (that appears in the Virus dialog if Swen is found) - it would not only delete the virus files, but of course also fix the corresponding registry entries - so the problems described above should not happen.