There are a couple things that it sounds like it could be. One could be that your OS was updated(like TPC was talking about) and at some point during the installation files were mixed up, this seems to happen fairly often, I mainly see it on people's computers that they do any sort of customizing on.
The second could be that your hard drive could have some failures on it, in which you would have to get your hard drive replaced.
Now this may be too technical for you, but if you want to try, download the Ubuntu LiveCD
, it is a Linux
OS that you can boot into at startup from your CD/DVD player or even from a Flash drive if you have a spare. So follow the instructions on that page to get your LiveCD/USB. Once you have that, go into your BIOS settings(usually you press Delete during startup of the computer, but can be another key) and get to your boot priority, and select your CD/DVD drive as first priority and then save your settings(many times it's F10) and reboot with the LiveCD in. Don't install when given the option, just boot into Ubuntu. Once you are in, let it load for a little bit, it might not be the fastest since it is loading off the CD. Once you have that up your hard drive should load onto the Desktop, open it and try to copy any important files onto an external hard drive or flash drive. Also, this is going to tell us if there are any real major malfunctions of the drive(it won't load properly if there are problems). Once you have all of the files off, I would suggest just using your recovery CD that came with your computer and reformatting and reinstalling Windows. It is a good thing to do anyway every once in a while because as you use Windows, it gets bogged down due to extra files that are no longer in need and a fresh copy without all of that is going to work so much better.
Hope that helps, let me know if you have any questons.