System restore is worthless. It was implemented back in the days prior to plug'n'play when you had to install drivers for everything. Sometimes stuff wasn't compatible. System restore was supposed to roll back these driver settings to a state prior to you installing the incompatible driver. They have added a few lame features to it since then but NOTHING useful. It does not remove viruses or recover your deleted files. Apple has implemented these features into their "time capsule". It's pretty much what system restore should have been years ago.

So this is a shot in the dark, but you could try using a data recovery program on the machine. It's much more complicated in your case b/c you're looking to recover *part* of a larger .pst file.