|Your first concern (naturally) is not to lose any personal files etc. So copy them to DVD at least and check the copies are accessible before proceeding any further after making the copies.|
Regardless of how you set about re-instalng windows (and sluc's approach is viable) first safeguard your files as above or below).
If you cannot boot into windows at present, then using another computer - download a free ubuntu ISO and burn it to a DVD. This is Linux based operating system. It will load itself into the RAM only - do not tell it to install itself to the hard drive. Once it boots up, to s typical desktop, you can locate your hard drive, and then copy the files to DVD that way. Again verify the copies are OK.
ubuntu.com is where to find the ISO (a file you "burn" to a DVD - not simply copy it).
Once your personal files are safely copied to DVD then you can set about rebuilding the system. There are a few options in that regard too... But "first" ensure your files are safe "off" the system as is.
You download the basic Ubuntu desktop version.
Again - before you actually set about re-installing windows - ensure you have copied your files to DVD at least.
You might be able to gain access to your windows system via safe-mode:
This would allow you to access and copy files without using ubuntu.
And this M$ article describes various options that may apply in your current situation.