I had a Windows Vista or XP machine and for some reason this key was not clearing after a reboot. So this error occured in a loop scenario. Error you may see in the event log:
Symantec Endpoint Protection (SEP) Installation fails with the following error “Symantec Endpoint Protection has detected that there are pending system changes that require a reboot. Please reboot the system and rerun the installation.”

The following was actually listed for a server 2003 box but worked fine for me on both Vista and XP.
I rebooted the machine and then tried to install the client again, but the same error would appear.

I found that there was a registry entry that was giving me grief with the install, and here are the instructions that I used to fix the problem.

1. Navigate to:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\SessionManager\PendingFileRenameOperations

2. Back up the registry key
3. Delete the registry key
4. Install the software without restarting the computer first (Restarting the computer may result in the registry key being placed back in the registry before installation.)
5. Restore the registry key from the backup location/ this is up to you. I did not restore it and worked just fine.

Enjoy