First, you may have some little programs loaded that are trying to write to the disk. For example, I recommend disabling the anti-virus program before running Scandisk or Defrag. I also turn off my Internet connection (it's always on when my computer is on) and even the screen saver. Check to see which programs are automatically loaded on your system by looking at the little icons on the bottom right corner near the time display. Some of them can be disabled by right-clicking on the icons.

I recommend running scandisk and defrag at least once a month. It helps to keep from developing so many lost clusters. I routinely scandisk and defrag 15 GB and 20 GB drives without any problems. The less often you run the programs, the longer it'll take.