Try booting to a different OS and rename all SAM.* files, then restart.
Windows should recreate the accounts and groups. You would lose all new
local accounts you set up. Move the hard drive to another Windows 2000 or
NT computer, for example, or just boot to a windows 98 floppy if the hard
drive is not partitioned as NTFS. floppy can be gotten from
www.bootdisk.com
If you don't know who deleted the accounts, the machine could have been
hacked, in which case you should consider formatting and reinstalling
windows and all other software, because other stuff may be wrong.
Investigating the machine first before you do that may make sense to
determine what was done and what security mistakes were made.
http://securityadmin.info/faq.htm#hacked