* Because your files will be replaced by new versions that you need the new software to read. - Utter rubbish, no they won't. Nothing is converted at all by default. Not your e-mails nor your documents.
* His Outlook-2000 email was reformatted to the new-and-improved Outlook-2003, and Outlook-2003 format is incompatible with everything except Outlook-2003 - Again rubbish the e-mail format in Outlook 2000 is identical in 2003. Plain Text, RTF or HTML, all standards. If you mean the PST format as another member has said it doesn't automatically convert this, you need to manually convert it. I run the old PST format in Office 2007 without any problems at all.
* Once Office-2003 has been installed, it can not overwritten with an earlier version of Office. Also, you cannot remove Office-2003 and re-install Office-2000, unless you know how to hack the registry - Rubbish it works fine, it sounds like a dodgy pc of which the Windows Installer Clean Up utility should solve the problem.
* And you can not easily install Office-2000 and Office-2003 on the same PC - Again rubbish, Office 2003 easily installs over the top of old versions, in what is called Hybrid mode.
Even if what you say is true I fail to see why a bit of Googling and registry fixing is slower than completely wiping the PC, and re-installing everything.
It's never wise to assume your problems are problems of the software in general.