I just got a new XP Professional PC for home use, and MS Word doesn't have the Helvetica font included (and has a lot fewer fonts than my work PC). Why?

You can always use Google to search for missing fonts, like what I've found:
http://www.fontseek.com/fonts/helvetica.htm
...and put the missing file in Window's Fonts folder.

One reason why some of the fonts are *missing* is that it's because to preserve/gain more computer speed.
I once did one really really bad thing - I downloaded and installed over 60 thousand fonts, guess what happened? First computer boot speed was 30 minutes, half of the programs, including Firefox, refused to work and crashed.. I had to get a Linux BootCD to erase and put default system Fonts back.

Currently I have 1100+ fonts, but already my 1.6GHz machine can't open Fonts folder anymore without crashing, but luckily boot speed and other programs aren't affected.

Try the open source word processor 'O O O' and Jarte Wordpad. Both are 1000 times better than the Windows products.

It might be because the pre-installed OS didn't bother to install everything. That is why I prefer do do my own installs; I can set things up to put in everything I need and omit crap I don't even want.

As for a lot fewer fonts, there is also the possibility that your work PC has fonts that your employer added. If I got a new car, I doubt it would have a skull-and-bones air freshener dangling from the mirror; that is an aftermarket customization. Ditto with many fonts.

It's probably on the installation disc. Use Control Panel > Fonts > File > Add new font. Hold down Shift as you insert the installation disc so that the installation program doesn't run.