I have just installed Windows 2008R2 on a HP DC7900 machine and had a lot of problems finding all the device drivers, even after installing EVERY fu….. driver from the HP download site it still would not find the last 2-3 devices (PCI something something)..

Well I found this on a HP forum and it helped me, download and install these two packages and rescan for new hardware (you may have to reboot also) and viola problem gone…;