Crooks can open fraudulent credit card accounts in your name with little more than a Social Security number and quickly run up huge bills that wreck your credit and cause endless hassles.

If your identity has been stolen -- or you're worried that a missing purse or purloined computer makes you vulnerable to such a crime -- the first thing to do is freeze your credit history.

That makes it impossible for banks or retailers to obtain copies of your credit reports from the three major credit reporting agencies: Experian, Equifax and TransUnion. Without a credit report, lenders won't issue a credit card in your name. (A PIN allows you to temporarily lift the freeze so a legitimate credit application can be processed.)

Consumers Union, the nonprofit publisher of Consumer Reports magazine, has created an online guide that provides state-by-state instructions on how to use those laws.

The credit reporting agencies voluntarily offer credit freezes in the five states that have not yet adopted such laws -- Ohio, Missouri, Alabama, Iowa and Michigan.

They'll also freeze the histories of any consumer who asks in the four states that only require such action for victims of identity theft -- Arkansas, Kansas, Mississippi and South Dakota.

Interest.com's identity theft checklist can make you a tougher target.