Grounded outlets were not required until the mid 70s. Your home may have been built with two-prong outlets, and a previous well-intentioned homeowner may have replaced the old two-prong outlets with newer three-prong version. However, unless he installed a ground wire, it is still a non-grounded outlet. Outlet testers are an inexpensive way to see if the outlet is wired properly. A tester can be purchased at most hardware and home centers.
If your outlet is not properly grounded or if it is an older two-prong outlet (Figure A) it would be preferable to have an electrician run a ground wire to the outlet.
Electronic equipment must be plugged directly into the surge suppressor with the surge suppressor plugged directly into a grounded wall outlet (Figure B) for full warranty coverage.