the first thing you should do is to flash the official updated ROM for your Mogul, if you are on Sprint you need this one:
http://www.htc.com/us/FAQ_Detail.aspx?p_id=75&act=sd
That is the official Sprint updated ROM for the Mogul which is the first level of GPS functionality. This ROM does not need any specific Bootloader, it should work with any of them. After you flash to the 3.xx roms you can use the 2.40 Olipro unlocker to allow yourself to Flash custom ROMS.
Go here:
http://forum.ppcgeeks.com/showthread.php?t=17667
This is the flashing guide for the Mogul.
As always, flashing ROMS removes all of your personal information, including Contacts, Calendar appointments, Tasks and Emails.
These items can be synced prior to flashing and then restored after flashing.
With regard to flashing on Vista, use the WMDC 6.1 and run your computer with UAC turned off and you should have no problems with the RUU (rom update utility) completing the flash. I generally use the SD Card with custom ROMs but if you use a kitchen or an OEM ROM you do need to connect your Mogul to a PC.