This is usually caused by other installation programs that registered their own version of ATL.DLL or other important DLLs, and then subsequently removed them. 

The automated way to fix this is:

1. Download the attached file att37.zip (very bottom of this page)
2. Reboot the machine
3. Using Winzip or the unzipping tool of your choice, locate & extract register.bat in the downloaded file
4. Double click the file.  Windows Vista Users should right click instead and select Run As Administrator.  You will see many "DLLRegister ... Succeeded" messages.
5. After all messages, your reports should work within the application.

Here is the complete manual method to install these files (you can copy the commands from this web site directly to the "run" box) :

1. Reboot the machine
2. Go to Start->Run
3. Type regsvr32 atl.dll
4. Click run or ok (whichever you have), you should receive "DLLRegister succeeded"
5. Go to Start->Run
6. Type regsvr32 urlmon.dll
7. Click run or ok, you should receive "DLLRegister succeeded"
8. Go to Start->Run
9. Type regsvr32 ole32.dll
10. Click run or ok, you should receive "DLLRegister succeeded"
11. Go to Start->Run
12. Type regsvr32 oleaut32.dll
13. Click run or ok, you should receive "DLLRegister succeeded"
14. Go to Start->Run
15. regsvr32 "c:\Program Files\Common Files\Crystal Decisions\2.0\bin\ExportModeller.dll"
16. Click run or ok, you should receive "DLLRegister succeeded"
17. Go to Start->Run
18. regsvr32 "c:\Program Files\Common Files\Crystal Decisions\2.0\bin\PageObjectModel.dll"
19. Click run or ok, you should receive "DLLRegister succeeded"
20. Go to Start->Run
21. regsvr32 "c:\Program Files\Common Files\Crystal Decisions\2.0\bin\ReportRenderer.dll"
22. Click run or ok, you should receive "DLLRegister succeeded"
23. Go to Start->Run
24. regsvr32 "c:\Program Files\Common Files\Crystal Decisions\2.0\bin\webReporting.dll"
25. Click run or ok, you should receive "DLLRegister succeeded"

We have also seen "Class not registered" when attempting to run ServiceLedger from a network share on a machine that has not had a proper installation to start.  ServiceLedger must be installed on a workstation from the installation program.

ServiceLedger provides the following link for those who must re-install Crystal Reports 9.0 run time to fix "Class not registered" in the event your issue is not solved by the above steps

1. Download http://support.serviceledger.com/download/cr9.zip
   (This install was created in 2003, and is titled "ServiceLedger 2003".  It will not, however, alter or install any other version of ServiceLedger; it only installs the Crystal Reports Runtime)
2. Extract the file using Windows XP or WinZip
3. Locate and double click "RunMeFirst", click Yes to import into the registry, you should receive a confirmation message
4. Locate and double click Setup.  Windows Vista Users should right click instead and select Run As Administrator.
5. When finished, reboot your computer

If you receive "80040154" or problems continue, you may wish to re-register atl.dll as shown in steps 1-4 in the top instructions if you did not perform it originally.  Some software installs this dll, registers it, then if you uninstall it later, leaves no registered version available.