For leaving or joining a domain you need some advanced user rights. And at least local administrator rights. Are you a local administrator for your computer? Also, for enabling file and printer sharing you need administrator credentials (local account, local administrator for that computer).
When you leave a domain (and join a workgroup), you may only use local users accounts (users that do not need to use credentials from a server to log on). Long story... Of course, if you know the account password.
It is your home computer? Or it is a work computer? If it is from work, talk with the network administrator (he will tell you some nasty things, he will reproach you...), he should be able to solve your problem.