Important: Prior to installing the DPM, make sure that you have a Domain User account that is already a member of Domain Administrators Group.You can install DPM by using a local administrator account. However, most configuration tasks require domain user privileges.

On the [Microsoft System Center Data Protection Manager 2007] screen, click [Install Data Protection Manager] [...]

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[Right Click] on the Command Prompt, then select [Run as Administrator]

  Open the [Server Manager].

Expand [Server Manager] to the [Roles] node, and then select [Roles].

Open the [Server Manager].

Expand [Server Manager] to the [Features] node, and then select [Features].

Firstly, remember that Microsoft encourage everyone to use a 64-bit servers to install the DPM 2007. But this doesn’t stop you installing the DPM on a 32-bit server.
Other requirements are as follows,

Here are the requirements that you should consider prior to install the System Center Data Protection Manager 2007.

You must select a single-purpose, dedicated server. Do not consider your Domain controller or any other Application installed server for this purpose.
If the selected server is a part of a cluster, then use the Cluster Administrator Tools to [...]