To make backing up your hard drive as easy as possible, you should set up a backup profile. Most PCs with Microsoft Windows XP have a program called Windows XP Back Up installed as a useful application. Once you open the application, you can follow the instructions from the backup wizard and decide what exactly you wish to back up.
To find the Back up tool go to: Start/All Programs/Accessories/System Tools/Back Up.
There are several third party back up programs available but for ease of use I would suggest you use the Windows one for now.
Buy an external hard drive large enough to take what you want to back up. A 500GB external hard drive would be sufficient to normal use. These can range in price from around £40 upwards.