The problem with the Net user options is that they require administrator rights to run. If they could be run as a standard user then anyone could use them to change the admin passwords or to enable the administrator, thus removing any security in the system.
as a last resort fix you could try this disk .... Offline NT Password & Registry Editor
You will need to download the ISO and burn this to disk using a suitable ISO burning program. This disk will allow you to rebuild your users
this is a Linux live disk that you should not use if you are at all unsure of what you are doing. read and print out the instructions and have them to hand during use as inproper use of this software will make you system unusable