Resey computer to default (by 3 users)

If this is an XP computer, you should try to login as administrator first. It's very common for this password to be left blank. Just enter "administrator" as the user and press enter. Re-installing the OS can be very time consuming, so this may be worth trying first.
See here for the procedure:
http://windowsxp.mvps.org/admins.htm
To reset" the computer. You'll need the disks that came with the computer. Either recovery disks or an OS install disks. This procedure varies from computer to computer, so I can't tell you much with out the make and model info.