Are you reinstalling the operating system?  Did you purchase the optional bluetooth?  What operating system?

I had the exact same problem after installing a fresh dual boot of xp and 2000. Installed all latest dell driver and utility downloads and still had unknown usb. Solved it accidentally by pointing to one of the device driver install directories, I believe it was the bluetooth but can't remember. Try a driver driver upgrade and point to the various driver directories created in the \dell folder, you'll find it. Sorry I can't remember exactly where I found it.

 

I just got a Latitude d600 and I also had the same problem, I did what you suggested, looked in the c:\dell folder, and it did install, but it ended up "finding" the usb 2.o drivers in the ati display driver folder which is somewhere between unlikely and impossible. I think what happened was for some reason windows xp doesn't install those automatically but it does include those drivers, you just have to manually find them, they are probably in one of the driver files included in the windows folder or i386 folder.

Be sure to check the Driver CD that is included with your machine. I had the same problem and loaded the USB 2.0 Drivers from the CD after hours of researching on the web.