That message is usually permissions related.
Are you logged in with a local Administrator account or a Domain Administrator account?
Can you try the following? Open up a command prompt using elevated permissions (Right-click on cmd and select Run As Administrator) locate the SP2 installer and run it from the command line.
If issue still persists take a look at the following forum.
http://social.technet.microsoft.com/forums/en-US/itprovistasetup/thread/61ce9777-aaac-41b5-ac02-eae76759d1d3/
MCITP: Enterprise Messaging Administrator 2007/2010 | MCITP: Server Administrator | MCTS: Windows Server 2008 Applications Infrastructure, Configuring | MCP | MCDST