Have you researched this at all on Tech Net? At the risk of duplicating information you already have, the drror code “0x800CCC0F” resolves to a dropped connection of some sort. The MS KB article here (KB 813514):
Are you running IIS? If so, this articlew may shed some light on what is happening (although it’s my opinion that if this was the case, all of your clients would be affected…):
I would certainly check any desktop firewall and/or antivirus routines and uninstal/reinstall as per the KB instructions.
I do not think this is a virus. If so, either the desktop or Exchange A/V program would alert when viral activity was detected (at least it will in Symantec Corporate 8.x – I’m not sure if 9.x will still do this, but would be surprised if Symantec removed this feature…)
Check your switch and/or router as well, to see if there might be some scheduled activity causing this service disruption. Possibly, it’s a flaky port (unlikely if no other program is affected, but not beyond the realm of possibility…)