Is it just a stand alone box? And what is your File system? FAT32 or NTFS?
If it is FAT, boot up with a 98 boot disk and browse to c:\winnt\system32\config\sam
Delete the SAM file.
The OS will then notice it is gone and replace it with the SAM in the Repair directory, which should have just a disabled Guest account and your original Administrator account, with the password you first defined.
If the File System is NTFS, you will need to download a version of NTFS DOS from the web, and use that to boot your system. After that, just proceed as above.
I'm pretty sure this will work, but I've never tested it, so it's obviously not 100% for sure.