acer desktop computer?

Can you start in safe mode?

Try booting from any live CD to get a go at saving your datas to some external drive.
If you can't retreive the datas from the drive, or the access seems flaky, the drive is bad or going bad: REPLACE it!

You may have some virus. It also may be a failing drive.
Get some antivirus like AVG or Avast and install it. Do a full scan and repair any problem found.
Both of those are available for free.

You very probably need to use another computer to get the anti-virus. Place the installer on a CD or USB stick and install from it.

If that don't work, or you can't install the anti virus, try booting with the install CD/DVD and try a repair.

If you realy can't repair your OS, you may need to reinstall.

If the repair or reinstall fails, this probably means that the drive is faulty or about to totaly die. You need to replace it and install your OS on the new drive.

Electro
November 2011