I have a Gateway PC - 507GR that is about 4 years old and lately it is going through periods where the Hard- Drive makes like a "downloading" noise , kind of like a crunching noise, and the computer slows down terribly. It can last anywhere from 5 to 20 minutes. This began a few months ago. What could be causing this to happen? Is the memory going? I do not have any viruses on it.