A system that is more than a few years old may be having an underlying problem with its hard drive. When you rebuilt the O/S you may have only done a quick format of the hard drive which wouldn't catch serious problems with the drive. A great program to test the hard drive is SprinRite by Steve Gibson over at http://www.grc.com. When I get a new drive I run SpinRite and try to run it once or twice a year on my drives to help solve these type of problems.
Looking for great gifts under $100?
Trendy tech gifts don't require a hefty price tag. Choose from these CNET-recommended useful and high-quality gadgets.