What os are you running? I actually just had this happened when I upgraded to windows 7. For me it seemed that a roboot didn't help but I didn't realize that Windows 7 was installing updates in the background. Since I only tried the roboot once and the updates continued after I rebooted it appeared that the reboot didn't help. In fact it did and once all of my updates where done installing the SEP install worked no problem. So you should check if windows updates are going on. If they are not I guess I would have to ask if you have had any failed installs recently. For instance you were installing iTunes and it failed half way or you shut down before that install was done?

Grant-

You many try this.
Open your registry, take a backup of the registry first.

no do a control+F  search for pendingfile

You will get atleast 5 keys , delete all of them.

do a system reboot

Now try to install should work fine.

Is actually a hook from MSI in the registry.  Something was installed (or uninstalled) with MSI and it probably didn't go as planned.

The MSI actually writes data into the pendingfilerename key to indicate that something needs to happen upon reboot.  Before it begins the install, MSI will check for this key, and, if found with data, will exit the "new" run of MSI, as there is something that needs done before another MSI can be run.

Removing the data from this key SHOULD allow another MSI to run (if not, try rebooting after removing it), but it's working around a potentially damaged install of some other software (which could even be SEP).  Think back...what was the last software you installed?  Is it working correctly?

You can also try using the MSI Cleanup Utility to try removing entries from the MSI catalog...although here I'm not sure how helpful it'd be.

(The MSICUU doesn't uninstall software, as is sometimes thought...it merely removes the reference to the software from the MSI catalog, thus allowing you to reinstall over the top, since MSI doesn't know the software exists after removing the entry from the catalog)

PendingFileRenameOperations can show up in a few places in the registry. 

HKLM\SYSTEM\CurrentControlSet\Control\SessionManager\
HKLM\SYSTEM\ControlSet001\Control\SessionManager\
HKLM\SYSTEM\ControlSet002\Control\SessionManager\
and
HKLM\SYSTEM\ControlSet003\Control\SessionManager\

PendingFileRenameOperations String needs to be removed from all locations.  Then you can reinstall Symantec Endpoint without restarting the machine.  The above post shows a photo of what it looks like. 

I've also found you can remove this key with a batch file.  A simple batch file like this can remove "PendingFileRenameOperations" from all the important locations in the registry!

@echo off
REG DELETE "HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\Control\Session Manager" /v PendingFileRenameOperations /f
REG DELETE "HKEY_LOCAL_MACHINE\SYSTEM\ControlSet002\Control\Session Manager" /v PendingFileRenameOperations /f
REG DELETE "HKEY_LOCAL_MACHINE\SYSTEM\ControlSet003\Control\Session Manager" /v PendingFileRenameOperations /f
REG DELETE "HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager" /v PendingFileRenameOperations /f

You could then script this to run on clients or an SMS job as well if needed.
 

Specifics can be read about deleting registry keys on this link. 
 

I've delete the values of all these and I still cannot install.  Really need some help.