You can take backup of your PC using linux live cd if operating system of your PC has failed to boot. This is very simple and no special expertise is required.
First try to recover boot problem, you can use recovery disk or your windows cd/dvd to repair start-up problem. In windows 7 it is mostly work. You can restore your OS using system restore option. When there is any update or new software installation/uninstallation a restore point created. You can restore using latest restore point to recover your PC. In windows 7 you can also make a system image and can save that to a external harddisk , in case failure of your OS you can restore your PC including all softwares installed, settings and windows update. You will not lose anything; see my previous post how to restore your PC from a system image in windows 7.
If everything doesn’t work than final solution is that you have to again installed your OS. If you have 2 or more partition and your data is in d/e drive than it is possible new installation will not affect on the data of these partition. Choose destination drive for fresh installation as C drive and you will lose only c drive data.
How to backup your data before installation – One option is that if you have some little knowledge of desktop/laptop hardware. You can detach your harddisk and attach to another PS using data and power cable and then copy your data to any external harddisk. You can also use some connectors that connect harddisk to usb port.
But the simplest one is that use any linux live CD to boot your PC, you can download at least one linux OS live cd ; ubuntu is the one of the simple and easy to use linux OS. Live CD means that you can access your whole computer using only CD, you can use internet, access all the files on your computer without installing it your PC.
You can download ubuntu from here and after downloading iso image burn it to a blank CD using your windows disk burning utility no additional software is required.
Chose in BIOS first booting drive as cd drive and boot your PC. Now connect an external hard drive and copy your all data to that drive.