How can I clear the Run history?

The Run history is stored in the registry in location 

as a series of values a-z. To delete an entry from the run menu perform the following:

1. Start the registry editor (regedit.exe)
2. Move to HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\RunMRU
3. Select the entry you wish to remove, e.g. h
4. Press the Del key (or select Edit - Delete) and click Yes to the confirmation
5. Double click the MRUList value and remove the letter you just deleted. Click OK to save the change
6. Close the registry editor