No need to replace the hard drive if the issue is a virus (help me out, other members: is there a virus that can damage the hardware?). If the laptop came with recovery media (a CD or DVD), and you are willing to lose all the data that was on your son's machine, just install a fresh OS off the media and then run Windows Update to catch up.
To answer your question: The Moraga Plus (?!?) is a small-form-factor SATA drive. If they consider 2.5" small-form-factor, new drive will fit. If the old drive is 1.5", no it won't. I don't see full specs on said Dell drive, which doesn't surprise me.
Edit: Ignore my second comment. I found a Moraga Plus as an EIDE drive here: http://www.shopping.com/hitachi-hitachi-40gb-moraga-plus-2-5/info
. So I can't tell you if it would work or not; you need the same kind of drive, and I can't tell what kind you have. Sorry.