No question is silly if one really wants to learn from it....
You should be able to replace the CD-rom drive with a DVD combo drive without too much trouble. They are modularized and as such should plug n play without issues....just remember that the DVD codecs will have to be installed in order to use the DVD rom portion....XP DOES NOT support DVD codecs on its own. So not only would you need to install the codecs but also a DVD player as well.
I highly recommend this stand alone player and the supporting codecs which will allow you to view all video types. Free-Codecs.com :: Download K-Lite Mega Codec Pack 3.7.5 : The K-Lite Mega Codec Pack includes the K-Lite Codec Pack Full, QuickTime Alternative, Real Alternative support and Monkey's Audio DirectShow decoder.
Be sure to add the Real Alternative and Quicktime alternative plug ins located on the left side listings.
About the interchangeability of HDD in your laptop. This also can be accomplished but with dificulty....Opening up a laptop is not the easiest thing to do as most of the screws and tabs are well hidden and things are easily broken especially the LCD screen hinges when a novice tries to open one up!
Like the optics drive the hdd is a modularized unit and interchangeable with most other laptop harddrives.