As far as the local Admin password goes, try going into the user applet in the control panel. Choose Administrator and click the reset password button. Enter a new password. Confirm it. Note however, you MAY have to do this with a user who also has Administator rights to the box.
To get back to the domain try this:
1) Open the control panel
2)Open the system applet
3)Click Computer Name tab
4)Click Change
5)In member of, click Workgroup and enter any workgroup name you want.
6) Click OK. You should get a message saying you are now part of Workgroup name.
7) IF asked to, reboot the pc
8)Go back and redone steps 1 thru 4.
Now choose Domain and enter the domain name or choose from the drop down list (if you are presented with one).
9) Again you should recieve a welcome to Domain Name message.
10) Reboot if asked.
You SHOULD now be back in the domain you were in orginally.