Optiplex GX 280/620 USB issues

Hi I am currently having an issue getting my Optiplex 280 and 620 to recognize any USB peripheral connected to it. We don't use the image that dell puts on the computer when we receive it, we push down our own image from a server. I built this image and made sure every software component was up-to-date, like windows up-dates, uninstalled/reinstalled USB root Hub ect. This problem is irregular, most of the time the image pushes down successfully without any problems with USB devices, however there has been quite a number of workstations where the mouse/keyboard will not work. And unfortunately with these models, there are no serial mouse/keyboard connectors on the motherboard, it's all USB connections(that confused me too). I was under the impression that Windows XP had a USB driver base, and you need only connect the device to the computer to have it auto-installed. For some of these workstations, this is not taking place (I had to establish a remote connection to the actual machine, checked the device manager and had to manually install the USB keyboard and mouse). I'm not sure if it's a issue with Dell, possibly my image or Windows XP. Any advice would be greatly appreciated. Thanks