I'm assuming you have an IDE hard drive, in which case take a look here:
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I recommend something like 80Gbs to 200Gbs, I've had the best experience with Seagate manufactured drives myself although WesternDigital is fairly good as well.
In terms of installing a new HD, it's fairly simple, just two cables to connect and you can't mistake em for each other
Once you have the new HD in you'll have to re-install Windows and all that on it, you can of course, connect the dying drive to it afterwards (make sure to always power off computer before switching components around) and make the new as Master (first cable connection) and the old on the seond connection and transfer files between them once you boot up from the new drive.