If you open a Terminal and type the command "man xpchelper" you will see the following manual entry:
NAME
xpchelper -- XPC helper agent
SYNOPSIS
xpchelper
DESCRIPTION
xpchelper performs services on behalf of the XPC IPC facility.
This command is not intended to be invoked directly.
This command is run, when Mt. Lion is installed, and is only to be run by system services, not directly!
The xpchelper cache is used by the xpcd daemon for XPC services, which undocumented. The file is in a property list format and contains entries for a variety of standard applications;
I tested what happens, when I remove the file. On none of my macs (two iMacs, three MBPs with different Intel processors) the removal if this file did cause any problems. Videos were playing well. Which makes me assume, that a healthy system should be able to run without this file in its cache. And if this is not the case, something must have been installed, that is corrupting the system library, so that Aperture has problems without this cached file.
That is why I am worried, that simply restoring the xpchelper file will only obscure one of the symptoms of the problem and not heal the problem with the system installation, so you may run into the same problem again later.
Reinstalling the system will be a safer solution in this case, even if it is much more trouble. It should suffice to reinstall on top of the current system - no clean install necessary. A clean install would defeat the purpose, because you would have to reinstall all your apps afterwards and probably reintroduce the very same problem by reinstalling the app that is causing all this.
Regards
Léonie