If you buy a new drive and ADD it to your existing one, it's fairly painless. Just follow the instructions with the drive (note for note), and you should be fine. (usually, you just change a jumper on the drive, plug it in, boot up, and format it).
Use the new drive for your recording space, and leave the rest of your computer alone. You'll have to tell your recording software to use the new drive to place your files, (usually just an option under "preferences", or something like it) and you're home free.
If you need to REPLACE your current drive, you'd have a lot more work to do. If you must go this route, and you're not confident with it, I'd pick up some drive software, like Partition Magic.
Good luck with it.