netlogon.exe process causing CPU %100

I am having an issue where our server really slows down.
After investigating I found that when the winlogon.exe
process is active, it consumes %100 of the CPU. It
remains active for about 5-6 minutes then it returns to
normal, for a littlewhile. It has just started happening
out of the blue and nothing has been changed on the
server. I have tried to reapply the service pack but to
no avail.

Any thoughts?

Thanks.