maxtor computer OneTouch4 USB not seen in My Computer

I’m working with a Compaq 6410nx pc running XP SP2. We purchased a 500gb maxtor computer OneTouch4 external USB disk drives to backup the system. When we plug in the disk drive, we hear the tones My maxtor computer associated with a new device being detected, but see no pop-ups that visually indicate that a new hardware has been detected. We have taken this drive and plugged it into other machines and it is discovered immediatey and asigned an available drive letter and is visible in My maxtor computer.
I have removed all the USB hub and mass storage entries under the USB Controller section in Device Manager and let the USB sections rebuild themselves.Upon looking in My maxtor computer, there is no drive letter associated with the new device. When I look in Disk Manager, I can see the device and it is indicated that the device is active and healthy. I have plugged in a Digital camera and a maxtor computer 512mb flash drive and both were assigned drive letters and were visible under. When I try to add new hardware through the control panel, If I reboot the machine, the assignment goes away and I am still sitting with a drive with an unassigned drive letter.
Just no Drive letter assigned to it. If I go and assign a drive letter to it, it does appear to be available in the drop downs and the maxtor computer backup progam can detect the device and run the backups, I get an error message though that indicates that another install is going on and I can’t search while that is running. Getting frustrated at this point and would appreciate any advice you could give me. Thanks for you time and help yet it is still not available under My maxtor computer.