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TESS VIGELAND: Last week we told you about all the scams proliferating online, on TV and everywhere else, promising to lower your property tax bill or help you avoid foreclosure. This week, the companies that claim they can clean up your credit. Sally Herships finds out why people are believing what they hear in those ubiquitous ads and what they end up getting.

Sally Herships: Beverly White is 58. She's divorced. Lives in Charlotte. She's a behavioral specialist with teen girls. She was renting a townhouse, but wanted to buy a house.

Beverly White: My goal was to own a home -- something I could show I had accomplished in my years of living.

But her credit rating was way too low to qualify for a mortgage. She had unpaid medical bills and other debt. Her FICO score was somewhere in the 400 to 500 range, or about 200 points less then what she needed. Then her real estate agent said he knew someone who could help. This guy could clean up Beverly's credit history. That was around August.

White: He guaranteed that by December my credit would be cleared and things would be OK and I would be able to then apply for my home.

You know where this is going right? December rolls around and Beverly's credit report: untouched. And the guy she paid 1,400 bucks to up front, isn't returning her calls. She's been scammed. The Federal Trade Commission says complaints about credit repair scams went up 50 percent last year. Tom Bartholomy is president of the Better Business Bureau in Charlotte. He says asking for money in advance of service is one way to recognize a scam -- it's illegal.

Tom Bartholomy: They make these promises, they make these guarantees and that's what entices people to come across with the up front fee.

But Bartholomy says scammers make claims people cannot resist.

Bartholomy: If you have a foreclosure on your credit history, they'll get that erased. If you have a bankruptcy on your history, if you have repossessions on your credit history, they'll have that erased. Those can't happen. It just cannot happen.

No matter how bad, how late, how scary items on your credit history may seem, if they're correct, Bartholomy says there's no way to erase them. Bad debts have to cycle off. And that takes time -- up to seven years. And scammers know people don't want to wait.

MY CREDIT NEEDS REPAIR.COM: Denied, denied, denied. Are you trying to buy a home? Is your credit causing your loan to be a problem? Stop being discriminated against because of your credit score. Credit repair services offer the removal of tax liens, bankruptcies, student loans and other collections.

That's a commercial for one of the many services that says they can remove negative items from credit reports. I wanted to find out how they'd explain this. Remember, unless there's a mistake, negative items can't be removed from credit reports. So I called one of the companies -- Today's Credit Solutions. I asked John Pautsch, a manager there, about a claim on their website -- a guarantee to improve scores.

Herships: That's something that you can guarantee for every customer?

John Pautsch: Oh yeah. For us the more negative items that are on there, the more we're going to remove.

Pautsch says they're able to raise their customer's credit scores by an average of 100 points.

Herships: Let's say that I really didn't pay a bill, how can you get that taken off my credit history?

Pautch: Just leveraging the laws to do it.

Pautsch says his credit repair firm is a member of the Better Business Bureau. But the BBB says they've revoked the company's accreditation. But, while almost everything these services claim isn't doable, they are right about one thing -- it is possible to clean up a credit score. Beverly White again:

White: The sad news is I could have done it myself from the beginning. Hindsight is always 20-20.

Tom Bartholomy at the Better Business Bureau says if there really is a mistake -- an error on your credit history -- you can contact the reporting agency to get it corrected. You're also entitled, by law, to one free copy of your credit report every year. And he says if you've had bad credit, you have to do what the lenders are looking for, show you can make regular payments.

White: I was really looking for a quick fix. That's what the bottom line actually is. A Quick fix.

Beverly White wasn't able to get a penny back from the scammer. But she says did get her credit cleaned up herself. She slowly rebuilt her score with new cards and quick payoffs. Now she says her rating is over 700 -- high enough to get a mortgage. She was finally able to buy a home in Charlotte -- her first.

I'm Sally Herships for Marketplace Money.