But it turns out these ARE quite valid reasons that I can hardly argue with. So in stead of fighting with him, I am actually sharing them here because these are the good points that should be considered pretty much every time when you decide to upgrade your computer, more particularly, your Operating System.
1. Stick with what you know
4. XP is still supported by Microsoft until April 2014
5. Developers still support XP
6. Upgrade components to stay secure
7. Do you really need to upgrade?
See, you can apply most of the reasons to other operating systems too.
[update on Feb 15, 2010]
Here also comes 7 reasons to move on in responding to this 7 reasons to stick with XP, from ZDnet as well.
And also check out the comments to see the 7 reasons not to use XP.