Also, most bios's these days have the "s.m.a.r.t" feature. But it has to be turned on in the bios. If it is turned on then smart should be warning you of drive failure to come.
If you dont have smart enabled then when you do go and boot up next time, go into your bios and see if you have smart enabled. Its easy to find in the bios.
As wolluf and firewolfrl have mentioned, you really should copy as much of your data as you can as soon as you can in case the drive dies as what you are describing is a dying hard drive.
If the hard drive does die on you and you havent copied your data and you want it badly, you could try the freezer method to bring it back to life again for a small period of time to try and get more data. If you want info on the freezer method just ask.
Good advice + great people = tek-tips