I believe that Flash uses the default windows temporary directory.. The only place I can find that setting is in the registry in:

HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\Curr entVersion\explorer\VolumeCaches\Temporary files

You could try changing that setting, and see if it works.

CAUTION: You are changing the Windows Temporary Directory.. This may have an adverse affect on other programs installed on your computer.. If you experience any unusual problems after making this change.. You can either try to reinstall the offending program.. or return the registry to it's original state.

Oh.. and whenever working with the windows registry, BE CAREFUL. I am not responcible for any damage to your system that you may cause whilest editing your registry.