A 750 watt PSU is fine for that system, especially one of the older PCP&C units. The weak link here isn't your PSU.
Run diagnostic software on your hard drive - I usually use GWSCAN, you can grab it from Gateway's support site, or it comes as a part of the Download Hiren
If the drive fails GWSCAN, it's a bad drive - yes, even brand new drives can be toast.
If the test comes back fine (run the full test - don't worry about your data - I've been using the software for 10+ years, never had a data issue) let us know.