During the installation of Microsoft DPM 2010 you will be asked to define a password for two local user accounts. The accounts are named MICROSOFT$DPM$Acct and DPMR$<machine name>.
The account MICROSOFT$DPM$Acct is used to start the services SQL Server (MSDPM2010) and SQL Server Agent (MSDPM2010) of the local SQL 2008 Express instance. The account DPMR$<machine name> is used for the Reporting services.
This post describes the neccessary steps to change the passwords of both accounts.