Based on the other symptoms you described, you probably have other issues. With certainty I can say that defragging the hard drive is the last thing you should be concerned about doing because its not going to make an ounce of difference in this situation.
First, you should check the hard drive to see if it is failing. If it is failing, defragging the hard drive with do more damage than good.
Take the opportunity while you can to make sure you have all the important data backed up. This is priority #1.
Then check the disk for problems. Click the Start button. Type 'CMD' into the search box. Right-click "Command Prompt" in the search results and run it as administrator. At the command prompt type CHKDSK /R and press ENTER. You'll probably be prompted twice. First, you'll be asked to unmount the drive, when you answer YES you will be told it can't be done and asked if you want to schedule the check during the next reboot, after you answer YES reboot the computer. Let the test finish. It will take a long time - several hours probably. There should be some progress indicated and problems will be listed as they are found. Messages indicating bad clusters signify physical failure or damage to the hard drive. Continuing the test after messages about bad clusters may do more damage, but at this point you have all your data backed up and you probably have to replace the hard drive anyways. Its also possible there are just a couple of bad sectors and CHKDSK will fix them and you will be on your way.
If you did receive messages about bad clusters, you should run the test again a couple of time over the next few days or weeks. Additional messages about bad clusters signifies hard drive failure.