HP DC5100...bad mobo?

Odd thing...load the OS (2K or XP, or an image provided by a client which includes the OS and all their supporting applications) and I can see the install detect the video, audio, and ethernet. Then it loads drivers, but at final reboot and ready for initial log on, the drivers are not present (confirmed by the device manager). I can manually load the drivers for all these devices using drivers I download from HP's site, and they will take. This is not my preferred method as the PC may later require another image, and I question if another tech would know about this glitch and be ready for it (as for the clients themselves, well let's just say they're not sure which end of the fork to stick in the toaster). Other DC5100s I've done do not exhibit this; I'm leaning on it being in the mobo, but what would allow a manual install of drivers, but not an automated one from the OS disks (which work on other machines)? PCs can be the cwaziest peoples.