The endpoint drive mapping feature on IAG is a functionality that a lots of administrators wants to use when configuring the IAG Portal due the transparency to the end user. However this functionality still not supported for Windows Vista users, even with IAG 2007 SP1. For more information on this read the IAG 2007 SP1 release notes:
There are no workarounds or fixes available for drive mapping on Vista to resolve this issue. As a possible alternative you can use the File Access functionality to expose the drive via the IAG File Access Web interface.