Possible error message you may see: Custom Form could not be opened

Clearing Cache:

The cache should be in Documents and Settings\%username%\Local Settings\Application Data\Microsoft\Forms. The "Local Settings" folder is a hidden folder. If it is not visible, you must go to Tools/Folder Options.../View and choose "Show Hidden Files and Folders".

NOTE: in Vista and above, the cache is in: C:\Users\<username>\AppData\Local\Microsoft\FORMS 

To clear the cache, close Outlook, then delete the Frmcache.dat file and all subdirectories and other files in this Forms folder. This is the same place in Outlook 2000, 2002/XP and 2003. When you start Outlook again, it will recreate the frmcache.dat once you try to access a custom form.