TO FORMAT, BACKUP DATA AND REINSTALL YOUR OPERATING SYSTEM:
basically what you'll need is:
an external source of data backup such as multiple DVDs or an USB hard drive.

backup WHATEVER you want to keep, pictures, music, anything, bank info, ect...
then your going to need your System Recovery disc, it shouldve came with your computer.

My system recovery disc for my old computer, (im running a custom build right now.) looks like this:
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If you did not buy your computer as a whole from a retail store or online or whatever then you had someone else buy it or it is a custom build.
if you have a custom build then you'll need a copy of your own Operating system. such as the installation disc for windows itself.

assuming that you purchased your computer from somewhere and it CAME with an operating system pre-loaded on it then you SHOULD have a system recovery, similar to the image above.

Once you have all the data you want backed up that you want to and your ready to reinstall windows, FIRST you will definatly want to disconnect any hard drive you have saved any data on then, lets get started...

First your going to put the CD into your disc drive and turn off the computer.

Next your going to start up your computer, normally the BIOS SHOULD be set to boot the disc drive before wherever your OS is currently installed (which for most people is the primary hard drive.)
if it just boots normal then what that means is the BIOS is set to boot the hard drive (or wherever your OS is installed.) as a higher priority than the disc drive, and we need to boot from the disc drive in order to continue.

If your computer boots into windows setup, do not do this step:

turn your computer on again and this time youll need to press a button to load a "boot menu" for me its F10.
it should say on your screen VERY soon after you turn it on.
once in the boot menu it should give you the exact name for your disc drive. depending on what kind of drive you have idk what it would be listed as, just keep booting into the boot menu and trying different boot devices until it loads from the disc drive which should bring you into windows setup.

End of that step.

once in the windows setup you'll need to format the drive, and reinstall windows.

i'm running off pure memory at this point so ill edit this if im incorrect on something.

you'll continue until you come up to a screen that displays your hard drive(s).

now youll have to create a new partition by clicking *New at the bottom of the window that is displayed.
it should automatically set to highest recommended size,
FOR EXAMPLE: if you have like a 120gb hard drive it may say 113gbs.

YOU NEVER GET ALL THE SPACE THAT YOUR HARD DRIVE ACTUALLY IS. EVER

it should ask you if it is ok to delete all the info because creating a new partition would erase the partition that is already on there (AKA Your data currently on the hard drive),

then your going to format the partition using NTFS format system, but you wont actually need to know what that is because it automatically does this for you when you select the new partition you made and click *Format.

once the partition is formatted, click next and it should automatically run the installation to install a fresh copy of windows on your computer. and at this point you can take a break until it finishes this process.

Once it reboots after installing it may ask a few questions such as:
what time zone your in, your desired Username and how many users you want on the computer, ect...

IF YOUR RUNNING A SYSTEM RECOVERY DISC IT SHOULD NOT ASK FOR A CD-KEY,
basically just put in whatever info it asks for and you should be good.

you should also have been give a Utilities cd, or something similar to such, this CD more than likely contains all the drivers and other software you may need so basically run that cd and install all the DESIRED stuff from it, some stuff on there is NOT NECESSARY, such as nero 7, which may be given to you but it is NOT required for your computer to function fully.

If you do NOT have such cd, then download this program called PC Wizard
heres a download link:
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run this program and google your:
video card for drivers
sound card " "
Chipset " "
And any other devices you may have troubles with, such as a PCI Internet card.

After you are done installing your drivers then you should be set and ready to transfer your data back."

OH and also make sure you update, considering system recovery cds are NOT fully up to date.