Credit history, whether good or bad, stays on your credit report for 7 years (exception is Bankruptcy, 10 years). You can always dispute anything that shows on your credit report and the credit bureaus will investigate it. While they investigate any disputes directly with the creditors that reported it, items being disputed will not show on your credit report for 30 to 60 days; or they may show as being disputed. This more or less cleans up your credit while it is being disputed and can improve your credit report and score for the moment. Once investigated (more than likely the creditors will have everything they need to win the dispute), if the dispute goes against you, the debts will show on your credit report and your score will go back to where it was or lower.
Credit Drs. basically dispute everything on your credit report. So your credit report can clear up for a month or two, and then go back to what it was unless a dispute was legitimate.
You can go to www.annualcreditreport.com, get your credit report for free and dispute info on your credit report yourself. I do not recommend this unless you have a legitimate dispute where something is reported incorrectly.
In short, do not pay for this service and do not make disputes unless there really is a mistake on your credit report.