Windows XP needs relevant package to support exFAT. Please download the package from the following link.
http://support.microsoft.com/kb/955704/
You can not format StoreJet to exFAT file system under windows XP OS before completing installing the package.
You can now format StoreJet to exFAT file system.
Please refer to the following link for more information about common operating systems and their supported file systems.
http://www.transcend-info.com/Support/FAQ-614