Actually on a workgroup or standalone machine you don't need to use the "computername\" prefix with the account name. You only need that on a machine that's a member of a domain when you want to tell it that you're logging on to a local account instead of a domain account.
Of course if the machine still THINKS it's supposed to be in the domain (i.e., if by "removed from the domain" the OP means it was disconnected from the domain network but not actually demoted to a workgroup machine) then yes, he will need to use the prefix.