Reformatting your pc without erasing the docs & files in your disk is a complex work. If your disk does not boot normally & you want to format it, just get another HDD (disk), make sure your new disk is not corrupted by viruses and have the OS installed already.
Set the jumper setting of your new disk to master, then your old disk (you want to fix) to slave, so that when your pc starts, your new disk will boot 1st and you can copy the important files & docs from your old disk to the new one. After copying remove your new disk from the system unit then return the original settings. You can now reformat your corrupted disk without worries because you've copied your files to the other disk.
After formatting set now your formatted disk to master then the other one to slave (vice versa) inorder to transfer your files to your original disk. But before doing this I reccommend you to install 1st anti-virus software so that when you copy your files, the virus that accompanies your files will be terminated to make sure your formatted disk will not be corrupted again.
Hope this will help you out a little... ^__^


have the OS installed already.

hi, i have issue with this part , you gave no instructions on install windows on the new drive ,a new drive will have to have windows install in your machine ,not come with windows allready installed ,because of chipset/ driver conflicts ,you will just get blue screen of death ,but your info is heading in the right direction .
There is simpler way and no need to buy a new hardrive,download knoppix in my signature ,burn to cd and boot to it ,it runs from the cd, you can then use a large capacity usb thumb drive [you need to right click on the usb drive and mount it ]and copy the file to it