I have noticed that some users are still could not get proper version installed for Vista 64 bit.
Please make sure that you download and install the version which says "Sohail's Gmail Notifier Google Apps - Vista SP1 & 64bit" If you find any problems after installation, please run this program "as administrator". It should work by doing that. Vista 64 bit is more secure OS than 32 bit so this program throws error if it could not get the access to registry.
Thanks