The official instructions from Microsoft to uninstall SQL 2005 Manually (http://technet.microsoft.com/en-us/library/bb418915.aspx) is difficult to follow and the way I describe is easier and more complete. This method worked for me because I wanted to go back to zero to start again. If you are not trying to do the same, I suggest you find another way. Follow these directions at your own risk.
Step 2. Uninstall from Add/Remove Programs. DPM always uninstalled fairly well from Add/Remove programs. SQL usually uninstalled but not close to completely. If one of the SQL components won't uninstall, don't worry we will remove it all through the registry. Restart after this step.
Step 3. Delete the DPM and SQL folders from wherever you installed them to. Usually at:
C:\Program Files\Microsoft DPM
C:\Program Files\Microsoft SQL Server
C:\Program Files (x86)\Microsoft SQL Server
Step 4. Open IIS Manager and under the Default Web Site, remove both Reports and ReportServer directories. Then go up to Application Pools and remove the ReportServer app pool.
Step 5. ****Warning: editing the registry is dangerous**** Some of this taken from Gilham Consulting wesite (http://www.gilham.org/Blog/Lists/Posts/Post.aspx?List=aab85845-88d2-4091-8088-a6bbce0a4304&ID=190).
Time to open Regedit.exe and delete some keys. Remove the following keys if they are still present:
HKLM\SYSTEM\CurrentControlSet\Services\MSRS 2005 Web Service
HKLM\SYSTEM\CurrentControlSet\Services\MSRS 2005 Windows Service
HKLM\SOFTWARE\Microsoft\Microsoft SQL Native Client
HKLM\SOFTWARE\Microsoft\Microsoft SQL Server
HKLM\SOFTWARE\Microsoft\Microsoft SQL Server 2005 Redist
HKLM\SOFTWARE\Wow6432Node\Microsoft\Microsoft SQL Native Client
HKLM\SOFTWARE\Wow6432Node\Microsoft\Microsoft SQL Server
Step 6. Do a search in regedit for the InstanceName$ of your SQL instance if you know what it is. If this is a Microsoft DPM server, your instance name will be MS$DPM2007$. If you find one and it is a value, not a key, press ctrl+tab to move to the key and then delete the whole key. If they are under LEGACY keys you won't be able to delete them and they are fine. There may be 4 or 5. Close out of regedit.
Step 7. Download and install Little Registry Cleaner (http://sourceforge.net/projects/littlecleaner/). Open this little app and let it scan the registry. It will find several leftovers. You can look through and select or de-select the ones you want. These are broken links that it finds and it should be safe to remove them all. Press the Fix button to have them removed. Restart the server.
Step 8. Your server is now free of SQL and you can begin installing again. DPM will make it past the pre-reqs for SQL now and as long as there aren't other issues not covered by this post, you should be able to reinstall DPM now.