"Administrator"
There is a fun little account on most computers running Win 2k and up. It's the administrator account, and it will give you unfettered file access, registry access, and network access, on most computers.
Ever boot up a computer, you just wanna check your email, and it comes up with a password screen, and you don't know the password because it isn't yours? Ever lose your password or want to bypass it on someones computer?
Use the administrator account! Win 2k users should see a screen at boot asking for a username and password (this is not the XP startup screen). Type into the username box "Administrator" and leave the password box blank. If the sysadmins have not been smart enough to lock this account down or delete it, you will gain access to the computer.
Win XP people should boot the computer into safe mode, preferably if you are going to check your email or use the network, select "Safe Mode with Networking" from the boot menu (access with F8 when you are booting the computer). When the XP startup screen comes up, vulnerable computers will have an "Administrator" account without a password that gives you access.
If the XP computer acts like a 2k (ie the screen isn't this big "WELCOME" thing, but a little window) type into the user field "Administrator" and leave the password field blank.
I know for a fact this works on 2k/XP, I think (but have never tried) it works on NT 4.0 as well.