Your hard drive may be in the midst of failing.
To verify this you have to look at the drive with another operating system and run a diagnostic test, especially looking for the [http://en.wikipedia.org/wiki/S.M.A.R.T.|S.M.A.R.T.] data stored on the drive. Two ways to do this:
One, Boot your laptop from a bootable diagnostic CD such as [http://www.hiren.info/pages/bootcd|HIREN] or [http://www.ultimatebootcd.com/|The Ultimate Boot CD]. Once booted, each has several HDD diag utilities.
Two, and my preferred approach, is to remove the drive from the laptop and attach it to another machine where you can run Windows-based diags, such as [http://www.hdtune.com/|HD Tune].
Most important here...do you have a backup of your data?