You can't open a shell? That's bad.
In a run box (Start Menu --> Run), type this:
If that does nothing, type this:
and then reboot your computer.
This will attempt to restore your damaged or lost system files from a local cache, or failing that, your Windows CD.
Then give cmd another try.
See here for some more info if you need it:
<a href="http://support.microsoft.com/default.aspx?kbid=310747">System File Checker</a>.