Problem with installing Debian AMD64 on hp workstation 6200

Hi,

We have several hp workstation xw6200 (Xeon 3.20Ghz HT, EM64T) and we
would like to install debian on them. I downloaded the
debian-amd64-netinst.iso and burned the image to a CD. The
installation stoped at the language selection screen: the PS2 keyboard
didn't respond (even numlock key).

The same installation worked fine with an USB keyboard, but the
problem occured again if I replace the USB keyboard by the PS2
keyboard after the installation: after OS boot, the keyboard didn't
respond at login prompt.

Sarge distribution (32bits) worked fine too.

Does anybody have an idea?

Thanks a lot