Sounds like the OS is corrupt; I'd try this:
- get a copy of JL_CMDER (search for it on these boards) and completely wipe out the BlackBerry. This will wipe out everything on the BB. I hope you have a data backup.
- Download the latest version of the 8700 handheld OS from T-Mobile at Unsupported Browser
- Execute the program you just downloaded
- Locate and delete VENDOR.XML on your PC C: drive
- Restart Desktop Manager; the OS should load now
- Restore data (if necessary)
Good luck and welcome to BB Forums.