... the tech walks in...

She looks over at the frustrated PC owner and asks him, "Mister have you spy, viral, chkdsk'ed, defraged and done your crit updates?"

PC owner looks around bewildered, "Uh ya I think so."

She turns and leaves while saying, "Don't call again until you know for sure."
MUHAHAHAHA

Ok, we had this same issue happening on my forums I run. There are a few issues that can cause it.

-Forums maintenace.
-Administration updating definitions to the software.
-ISP on your side not resolving,
-The domain provider is doing something whacked on their side.
-Spyware/viral/firewall changes
-MS crits changing something.
-Someone else logging into the same forum with a different user. This problem is common in a multi user household all using the same User/Log-on to the computer and your computer goes "UMM WAIT! Who is this!" This was referred to as "Cookie confusion" on my forums LOL Windows XP crits did fix this a while back though.

The most common problem is the domain provider and the issue usually clears within a day or so. We had this problem with ours and I was on the phone with the domain provider for an hour until they figured out it was THEM and not something I had done to the site. However, doing a good maintenance check on your system is always a grand idea anyway.

I know the thread is a tad old by a eight days, but maybe it will help someone else