I got this from the Blackmagic design's support section on there website. This sounds like the problem you are getting.
HP xw8400, xw8600 and xw9300 configuration
Before installing a DeckLink, Multibridge or Intensity PCI Express card in an HP xw8400 Workstation or an HP xw9300 Workstation, you will need to carry out the following procedure. This procedure will avoid the computer from unexpectedly rebooting, or encountering a blue screen, during start up.
1. Ensure that neither the Blackmagic card, nor its drivers, are installed in the system.
2. Switch on the computer and press F10 to enter the Hewlett-Packard Setup Utility.
3. In the Hewlett-Packard Setup Utility, go to the Advanced menu and choose Chipset/Memory.
In the ensuing dialog box, set the options as follows:
PCI Express capture cards from Blackmagic Design are compatible with the HP xw8600. The HP xw8600 has PCI Express slots which can be reconfigured in the BIOS and these changes can lead to compatibility issues. It is therefore important to review the following information to ensure compatibility.
Expansion slots 5 and 6 of the HP xw8600 are mechanically the size of an x8 lane PCI Express slot. Slot 5 is electrically only a x1 lane slot which can be reconfigured as an x8 slot in the BIOS. Slot 6 is electrically a x4 lane PCI Express slot.
Blackmagic PCIe cards are compatible with the HP xw8600 if the PCIe cards are installed in slot 6 of the HP xw8600. A Blackmagic PCIe card will not function if the card is installed in slot 5, even if the slot has been configured as an x8 lane slot.
Blackmagic Design recommends that slot 6 be used with DeckLink, Intensity and Multibridge products. Slot 5 should not be used with PCI Express cards from Blackmagic Design.