If you are in a situation that you need not want server to be rebooted after SQL Server 2008 R2 install,then follow the below steps

You can bypass the “Restart computer” rule when installing SQL Server 2008 r2 and work around this by following these steps:
1. Open Registry Editor. Click Start –> Run, input “regedit” and press Enter.
2. Go to HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager, and delete any value in “PendingFileRenameOperations” key.