Log in to the CMC and under Folders locate one of the reports you're having problems with. Right-click on the report and select "Database Configuration". Make sure that you have the correct log-in information configured here. Also, in the "When viewing or scheduling report" drop-down at the top of the screen, make sure that you've selected "Use same database logon as when report is run".
Another thing to look at - are you using dynamic parameters in your report? If so, you'll need to log in to the Business View Manager (BVM - it's part of the Client Tools install), find the connection ("_DC" object) for the parameter and edit it to make sure that the correct credentials have been entered and that it "never prompt"s for a login.
-Dell