Download Malwarebytes from http://www.malwarebytes.org/products/malwarebytes_free and run a full system scan to eliminate malware as the cause of the problem.

All Windows operating systems have two administrator accounts. This isn't unusual. However, the system wouldn't be able to switch accounts in the way you describe. So either there's some third party user intervention happening somewhere, or the operating system itself is corrupt.

You can also install this free utility to check whether your machine has been compromised and has become one of the millions of home user systems to form botnets: http://free.antivirus.com/rubotted/ (explanation on the site).

I downloaded the two you advised. I had 65 Malware problems. Bots = Zero. I won't know if this helps until I use my computer for a bit. Thanks.

It didn't help at all. The User names switched by themselves, again.

Are your administrator accounts password protected?

Also, do you have Fast User Switching enabled? See How To Use the Fast User Switching Feature in Windows XP

Sorry, the previous answer did not solve my problem. I hit the wrong key by mistake. The 'switch faster' was not on, but when I turned it on, it didn't help.

OK, not to worry.

After you removed the malware found by Malwarebytes (I presume you did), did you reboot afterwards?

Try disabling Fast User Switching in services. Instructions on this site: http://www.howtogeek.com/howto/windows/disable-fast-user-switching-on-windows-xp/

Yes, I rebooted, and have disabled fast switching.

This didn't help me.

Can you do the following please.

Click File | Work Offline. This means that Fireox no longer has Internet access and can only be used to browse your history links.

But does it switch accounts in that configuration too?

Click "Work Offline" again to disable it.

Where do I get 'work offline'?

Link on your File menu. See screenshot.

There isn't a 'screenshot' there.

Mozilla seems to have screwed up the menu in the 'new layout'.

Please do the following. Right click a blank part of the Tab bar and then click the top menu called "Menu Bar".

You should now be able to click File and go to Work Offline.

I have a small home business and need to be able to work Offline.

Yes, I understand that, but we're trying to eliminate outside interference as the cause of the problem. It would be interesting to see whether or not this mysterious switching of accounts only happens when you're online, or whether it also happens offline.

When you say 'new layout', are you talking about Version 4? Because, if you are, I had this problem for the past year.

I found the 'offline' key and pressed it. It won't let me get into the computer, so I can't try anything to see if it makes the problem go away.

You have two options in Firefox 4. You can have the layout with the oblong orange button top left, or you can make FF4 look like a previous version by enabling "Menu Bar". I prefer the latter. I've also moved tabs below the location bar. But this is personal preference.

However, I reconfigured it to the FF4 default format and noticed that there isn't an option to Work Offline for some strange reason. So just to test whether this mysterious problem you're having, I suggested reverting to the old layout as can be seen in the top half of the screenshot.

Or was that not what you were referring to?

Could you do the following please.

1. Press Windows key + E to open Windows Explorer.
2. Right click your C: drive and go to Properties.
3. Click the Tools tab.
4. In the menu at the top, click the Check Now button
5. In the next menu which appears, checkmark Fix File System Errors
6. A message will appear that the drive cannot be checked because it cannot be locked and do you want to schedule a check on the next reboot. Click Yes and then reboot.

I couldn't locate what you said to do.