I had the same problem with my Esprimo 5545, both on XP Professional and Vista Business X64:
This happened for the first time only three days after the purchase during the system set-up, and at that moment I referred it to drivers and/or system instability. Then, after nearly a month, the same problem repeated again rapidly in a few occasions...
Finally, after I've reported the problem to Helpdesk, FSC Service replaced the motherboard, and I haven't experienced any problems since.