Constant disk access, often called "disk thrashing," is usually a symptom of insufficient RAM, heavy disk fragmentation, disk failure, or a virus/spyware infection.
The first thing I'd do is a simple SMART diagnostic on the drive to determine it's health. You can perform this with software such as Speedfan or HD Tune.
If the hard drive SMART status is good, then it's more likely to be one of the other problems. How much RAM does the system have? What does Task Manager tell you about the software running on your system? When's the last time you ran a full system scan for viruses and spyware?