http://www.iainkick.com/archive/2010/08/the-dreaded-uac-and-ssrs-2008/

So simply right clicking on Internet Explorer (or your alternative browser of choice) when opening the browser and selecting ‘Run As Administrator‘ gave me access to the Site Settings and other links in SSRS. Also running BIDS as Administrator made sure the deployment was successful. We were then able to add users to SSRS and they could access reports as normal.