Good to know about the Network Access Account in SCCM 2007

The Network Access account is never used as the security context for running programs, installing software updates, or running task sequences, only for accessing resources on the network.

Required rights and permissions
This account should have the minimum appropriate permissions on the software distribution or operating system deployment content it needs to access. The account must have Access this computer from the network on the distribution point or other server that holds the package content.

The program is always run in the context specified by administrator in the program properties. The Network Access account is never used to run the program, even if it was used to access the distribution point shared folder.

http://technet.microsoft.com/en-us/library/bb680398.aspx