When installing the rsSharePoint.msi on Windows 2008 you need to ensure you run it with "Run As Administrator".
Having said that, in multi-machine scenarios you may run into an error along the lines “Insufficient SQL database permissions for user ‘SYSTEM’ in database Sharepoint_AdminContent_<guid>….". The root cause, is on Windows 2008 or higher version OS's rsSharePoint.msi run its custom action (rscustomaction.exe) to install and active the RS feature in SharePoint as SYSTEM, which is known to fail in multi-machine scenarios.
Here is the means to workaround this issue:
We are currently working on fixing this issue in rsSharePoint.msi within the SSRS 2008 R2 timeline.
HTH
Prash