3700...hmm. Well i have a 4600 and i replaced my drive with one from a WD external drive. its actually really easy. As for knowing where the drive is? I'd just open the thing up. you're not gonna get your self into trouble just pulling screws. My drive was right there under the main panel and was held in with a few screws.
If you don't want to lose any of your data you can either back everything up, or just ghost everything onto the new drive then put it in. That's what i ended up doing.