You are likely to have to do a complete re-install of Windows 7
http://www.canadiantech.info/for-techies/re-build-windows-7/
If you are going to do a re-install, you may be able to take your data off first. Remove the hard drive and put it into a USB external drive housing. Connect to a working PC, copy the data off.
Always check to ensure the hard drive you are about to rebuild is actually sound from a hardware perspective. Download and run your hard drive manufacturer’s drive testing software.
First off, you want to try a System Repair disk to boot the PC. It may be more successful in fixing and offer other alternatives. If you do not have one, go to any other PC that has windows 7 with the same bitness (32/64). Start globe, All Programs, Maintenance. You need just a blank CD. When you boot, ensure that you are in fact booting on that CD and not the hard drive. Usually pressing F12 will lead you to the option to do so.
If that does not work, you are likely into a re-install. Takes a lot of time, but not all attended time.
- Factory restore — replaces everything on the hard drive with exactly what it had on it when you bought it
- “Upgrade” install using the Win7 install disk. You start as if you are re-installing from scratch but when it gets to the point where it offers the Upgrade choice you choose that.
- Full Clean install. The only absolutely sure way to have a perfectly clean system.
You will need a Win7 install disk for 2 or 3. You will need a legal product code for 3 and the win7 disk must match the edition of the code and its bitness (32 or 64)