if your Dell DJ player is no longer recognized by the upgrade firmware even with the correct, original device driver installed, then you will need to install a windows 'MTP' driver by doing the following. Note that this applies to Windows XP only.
1. make sure you have the SP2 XP upgrade.
2. Install Windows Media Player 10 or roll it back to version 10 if you are using a later version. You can roll it back by going to 'Add or Remove programs' and uninstalling any WMP upgrade from the 'Change or Remove Programs' dialog window. Note: don't uninstall WMP enitrely from the 'Add/remove windows components' dialog.
3. Go get this file: fixup.zip
Unzip it in a temporary directory and run the fixup.cmd by double-clicking in explorer or using the windows 'Run' command. Follow the instructions it prints in its DOS window about deleting the Dell device in the hardware manager.
4. plug your DJ player into the USB port. XP should now say 'Found MTP device' or something similar.
5. Get the Dell DJ 1.02 firmware from their support site appropriate for your player. Run the firmware upgrade. The software should detect your player this time.
A good reference for the Dell DJ stuff is here: gotzune.com
Note: I can't personally recommend the 'PlayForSure' firmware as I have not been successful upgrading to it. WMP 11 does not seem to work with the Dell DJ but WMP10 does and the device Sync function works just fine.
For almost ALL of the MP3 players out there that seem bricked due to a firmware upgrade failure or some other reason, they can be usually restored in a simlar way by installing a basic 'MTP' driver which allows windows to at least send a file to it, and by putting the player into a recovery mode as defined by the manufacturer.