Just alter the jumper on the back of the drive to the 'slave' position. Open the computer's case and install the drive in a spare drive bay. Connect it to one of the spare power leads that you'll find waiting, and also to the available connector on the IDE cable that runs to the existing drive.
Turn the computer on, and wait for it to boot into Windows on the new drive. With luck you'll find that Windows has detected the slave drive, has allocated a drive letter, and is displaying it in 'My computer'. All your data files will now be accessible, and once you've made sure that everything is working properly you can erase the old Windows operating system from the slave drive.