For the last couple months I’ve been troubleshooting a vexing problem on my new quad-core HP desktop: Roughly once per week, the machine would start running as slow as molasses. And I mean slow: ...

Q. When I defragment my C drive, the graph goes only to 10 percent before I get a message that says the drive's content changed. How can I correct this problem? Q. When defragmenting, drive C gets to ...