My Mac is trying to log into my school's domain w/o authorization?

Hello everyone. I was sitting in the tech room (in this situation it's a good thing I work for computer services at my school, otherwise they'd probably think that I'm doing something malicious) when the network admin came up to me asking "Isn't your computer's name Lappy486?"
Me- "Yeah"
Admin - "It says your computer tried to log into the domain server as 'guest'"
Me- "Uh...sweet?"
Admin - "No, now I have a reason to say Mac's are stupid and unsecured, your computer must be being used by someone else to hack the network1!11!!"
Me- "That's really unlikely, I bet my mac is just somehow detecting the domain and trying to pair with it because Mac's are sweet and like to go out and do such things automatically"
Admin - "pshh"

Our domain is using Windows Server 2003 and when we checked the logs one other mac on the network was shown to do the exact same thing, attempt to authenticate with the domain as a guest. I'm sure someone knows what I'm talking about.

Sigh,

Now there's all sorts of suspicion surrounding my Mac in our department. As a first time Mac owner and previous mac hater, I know exactly what they're all thinking. Someone help!

I just realized there's a "networking" section in this forum. Sorry everyone.

I'm moving it now.