This problem is usually caused by corrupt files in Windows usually caused by a crash or sudden shut down (like a power-outage.)
There is some slight possibility that your RAM is failing, so try http://windows.microsoft.com/en-us/windows7/diagnosing-memory-problems-on-your-computer.
If this is not a problem, you may (or may not) have some luck if you (i) disconnect everything except monitor, mouse and keyboard (ii) open Windows in Safe Mode and then (iii) see if you can access a restore point before the disaster.
If this fails, run the command checkdisk /r from the command prompt (see below and http://kb.wisc.edu/page.php?id=6565
which is valid for Win Vista/7) Type cmd in the start menu and right click on cmd.exe. Then select "Run as administrator"
If that fails,
a)Insert the Windows 7 installation disc.
Note: The computer must be configured to start from a CD or from a DVD. For information about how to configure the computer to start from a CD or from a DVD, see the information that came with the computer.
b)Restart the computer and boot from the Windows 7 installation disc.
c)Set your language preference, and then click on Next.
d)Click on Repair your computer.
e)In the System Recovery Options dialog box, click on the operating system that you want to repair, and then click on Next.
f)In the System Recovery Options menu, click on Command Prompt.
g)Type in chkdsk /r (note the space between k and /) and press enter. This might cause a message that checkdisk cannot run and ask you if you want to have checkdisk run when the computer next starts: type yes and then restart the computer.
h)When the repair process is complete, type exit.
NOTE: While performing chkdsk on the hard drive if any bad sectors are found on the hard drive when chkdsk tries to repair that sector if any data available on that might be lost.
If this fails, and you have no back-up, then you are in "reinstall Windows" territory. Buy a new HDD, remove the old HDD, install the new HDD, format and install Windows on the new HDD. Now connect up your old disk and, with luck, the new disk will be able to see the old one.