Because you can't use net user command and return error 5 this mean you have been removed from Administrators group.I don't know it is worth to try
neverless:
Try to run system in safe mode and log-in to buildin administrator account and try add your account to administrators group.If buil-in admin account not appear in safe mode (if something wrong happend with the system this is possible) try to enable buildin admin by using one of them methods:
METHOD 1
User Account Password - Change from Win RE
METHOD 2
http://www.vistax64.com/851487-post2.html http://www.vistax64.com/331839-post10.html
METHOD 3
Tools for reseting password also have option to enable/disable accounts (for built-in admin also) for example
http://home.eunet.no/pnordahl/ntpasswd/
EDIT
And one question.How did you try restore system undo? From system level or from Vista DVD?
System Restore - undo
METHOD ONE from this (from Vista DVD) should help although there is ability Vista DVD can reject restore system also