I don't suppose this has anything to do with trying to "hack" the device to go higher than 999 MPH or 99,999 ft does it?
Garmin Handheld GPS Question
If that is the case, I don't think this is the sort of thing covered in the warranty.
It sounds like you really screwed up the ROM. Unless it is an EEPROM I don't think you can fix it yourself. Even then you need special tools and the original software the device came loaded with. Try to call Garmin and tell them what happen, leave out the part about trying to hack the device and hopefully they will take it back and repair it for you. If its still under warranty then the repair should be free but you probably have to pay shipping.
[Edited 2006-07-27 15:30:39]
[Edited 2006-07-27 15:31:17]