First, try logging in as Administrator with no password. Also try iterations of the words password and Administrator: password, Password, P@ssw0rd, password1, Password!, Administrator, administrator, Adm1n1str@tor, etc. It seems stupid, but many adminstrator passwords are easily breakable.
If the Win 2000 installation has a FAT32 boot partition, you can reset the password with a Win98 boot disk. Boot to DOS with the boot disk and delete the SAM file in the \windows\system32\config folder. If you delete it, you'll delete all local users with it. Also, any files and folders that were encrypted with those local accounts will be unrecoverable. After deleting the SAM file, you can logon as Administrator using a blank password.
Finally, there is a great utility that will work with both Win2K and WinXP. It is the NT Offline Password Recovery tool. Go here to learn more:
http://home.eunet.no/~pnordahl/ntpasswd
The actual utility can be downloaded at this URL:
http://home.eunet.no/~pnordahl/ntpasswd/cd040818.zip
On a computer with a CD burner, double-click the .iso file and it will create a bootable CD for you.