Verify you COM+ rights for the DPM agent. It could be that there is a missmatch in the rights.
Click START / Administrative Tools / Component Services
Expand Component Services / Computers / My Computer / DCOM Config
Right click on the "DPM RA Services" and choose properties. Verify that under the Security tab / Launch and Activation Permissions you got Customize marked, click the Edit... button in the ACL you should see the computer account for your DPM server. If it's not present add it and mark all allow boxes and try to start the service again.
