I would add another hard drive to it, and buy a big one like 100+GB. Once you install it, it will detect it automatically and will appear as drive E: or somethng like that depending on what other devices you have in that machine.
Probably the hardest thing to installing a hard drive is having to deal with the jumpers. Don't get me wrong, it's not hard once you know the understanding of it. You're probably using an IDE hard drive so there's some configuring you will need to do. SATA drives have nothing to set. Literally plug and play.
On the back of the hard drive, there's a slot that has 8 pins in it and you'll see a small black plastic "jumper" that basically connects a pair of those 8 pins together. That's why they're called jumper so the current can jump across each other. Usually, they are labeled, Master, Slave, and Cable Select. Here's a site that can help explain what each does:
That's for a Seagate hard drive, but other manufacturer may vary but the meaning is the same. Simplest way is to set the new hard drive as Slave and install it on the primary slave plug on the cable.