try C:\Windows\ehome\ehshell.exe not a stupid question it's really intuitive, thanks MS

This is what Technet has to say about the error

Users receive a message that says "Windows cannot open this program because it has been prevented by a software restriction policy. For more information, open Event Viewer or contact your system administrator." Or, on the command line, a message says "The system cannot execute the specified program."Cause: The default security level (or a rule) was created so that the software program is set as Disallowed, and as a result it will not start.
Solution: Look in the event log for an in-depth description of the message. The event log message indicates what software program is set as Disallowed and what rule is applied to the program.

Interesting being that home premium dosn't have a security policy editor to change a disallowed entry back looks as if something has changed the registry entry that relates to this "Policy"

I could be DEP acting on a Codec you could try switching off DEP to see if that clears the problem.

Disable DEP in Vista:

1. Click Start
2. Click All Programs
3. Click Accessories
4. Right Click on Command Prompt
5. Left click on Run as Administrator
6. Click the Allow button if it asks you for permission
7. Type or copy and paste this into the command prompt window:

bcdedit.exe /set {current} nx AlwaysOff


8. Hit enter and you should see the confirmation: Operation Completed Successfully
9. Reboot and DEP should be off.

if it is DEP then it's time to start disabling codecs untill you find the problem or if your lucky you may find the culprit in the event log as per MS answer

sorry can't be any more help