I have a good 40 gigs I'd want to back up.
Buy 3.5" hard drive. Make sure that it conforms to the interface that your computer uses (SATA, or IDE/PATA) Set the Jumper supplied with the drive to the setting that allows it to be used as a non-bootable slave drive. Open your computer case, ( Make sure that your computer is OFF when you do this. Also, make sure that the new hard drive has at least the capacity of the information that you want to backup, more space is invariably better. -- You may be able to get a used drive cheaper than a new one.
If you have space in your computer case, place the hard drive on end, with the interface facing up, or lay it down on the bottom of your computer case, making sure that the PCB on the hard drive is not touching anything. unplug the data cable from the hard drive that is already mounted in your case, and plug it back in, ensuring that the data interface socket on the end of the cable is free. --Plug this into the Hard drive that you have just purchased.
Also, if you have no extra IDE PSU cable in your computer case, you may need to get one. If you need to get one, look at the motherboard in your computer for the proper minisocket and plug the corresponding end of the cable into that, and the other end into the power socket of your Hard Drive.
Boot your computer. -- If you are running windows, it will recognise the new hardware, and give your hard drive a corresponding drive letter. You may now back up your files to your new drive.
After backing it up, you may want to disconnect the cabling from the drive, and place it back in it's antistatic bag and store it until after you wipe the existing drive and reinstall the OS. PLug it back up after you reinstall Windows, and then copy the information from it to your old drive.
--If you copy the info instead of transferring the info, you'll still have a backup available if you fuck up your original Hard drive again.