Check Disk
This is the same old problem since the beginning of hard drives. Hard drives suffer from bad sectors and cross linked files.
Think of your old VHS tapes. When the tapes would get old they would start to show white dots on the screen when the data is missing from the tape.
The same thing is happening to your hard disk. Over time it will slowly lose its ability to hang on to your data. There is nothing you can do about this because it is the nature of how hard drives work. What you CAN do is watch your drive on a regular basis and "catch" your data before is get swallowed up in a bad sector.
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1. Open MY COMPUTER
2. Right Click on your local disk (usually drive C) and select PROPERTIES in the popup window
3. select the tools tab and press CHECK NOW in the error checking section.
4. select both options ("automatically"... and "Scan for"...)
5. say yes to the exclusive rights message that pops up. Windows needs exclusive access so it schedules itself to run this when the computer is rebooted next.
You don't need to worry so much about DEFRAG so much these days. Sure it will help a little but the secters on your disk are so small
these days that it simply isn't as importand as it used to be.
The computer is not going to stop because it is fragmented but it WILL stop if one of your key system files falls into a bad sector.
Run error checking about once a week and always after a crash or power failure.