Windows XP Auto Log Off????

Here's a problem I have been having, maybe someone can help me figure it out:

I never log out of my computers, always use the Windows Key + L to just send my comptuer back to the login screen but keeping me logged in (switch users feature). Because I always keep my chat client, Trillian, open and leave it run all day. But still like to keep my computer locked.

Now the funny thing is, now and then I will notice that my computer has been fully logged out. Never restarted (I've looked at the logs) or anything, but when I log in it plays my "starting windows" sound instead of my "login" sound... and none of my applications I was running will be running.

Before now I never really cared much, because this happened very rarely. Though I just formmated two weeks ago, and as of a week ago my computer is always logged out when I come back to it. I don't understand why, is there a setting that I forgot about or something? I never remember changing a setting before about login.

I've also noticed this happens on more than one computer, with the logging off. But like I said it's a rare thing. Though on my main computer I use, like I said it's been happening like everytime I log off and walk away from my computer and let it set for longer than 15 minutes (I'm roughly assuming that long).

My power management is setup to not shut any of my devices off, I unchecked the box to ask for a password on resume (thinking that might have been the problem), and if any of my power buttons are pushed on the case it is suppose to ask me what to do. I have no screen saver set either, I just simple turn the monitor off when I walk away from my computers.

Is it me, anyone else have this happen, is it a setting, can anyone give me any clues or maybe I could check things that I'm just overlooking?

Thanks for your help!