Dell 4100 with W XP home, locked out!

Hello,
My wife used our desk top Dell 4100 today to download a menu from Red Lobster, and left the computer on while we went out for Dinner with friends. When we came home after five hours, the screen was blinking blue, then wall-paper, then blue, et cetera. I picked up the mouse, and when I did, a message came on with the Windows Xp Home edition banner saying " This computer is in use, and has been locked. Only (my name / my wife's name) or an administrator can unlock this computer." It gives her name, and asks for the password. My problem is that I have absolutely no idea what the password is, as I have never used it! The computer came with Windows ME, and I had it upgraded by Best-Buy to XP Home, so if there is a p/w, then they put it in and I have never needed to use it! What do I do now? Is there a standard P/W used in these cases, or a way to reset one. I have my original XP box and instructions for installations, but don't see anything in the literature for this problem! My wife is now giving me dirty looks because I merely asked her, what she did to my computer! Boys, I'm in the doghouse now. Thanks for any help.