Actually if he set it up properly (with a spare partition) for a dual boot, yes he could add Win 7 at virtually any point he desired. AND maintain the ability to go back to XP.
this is a great route to take....... ( or at least plan for by making "2" partitions before loading the WinXP OS, and keeping the 2nd partition for future instal of win 7 64 bit OS )
it even saved "me behind" this past weekend, I was cleaning up some folders using the "DOD 5 times stripping /erasing method" ( <--- or whatever it is going by memory ) and I was like only 1/4 way awake friday morning... I accidently highlited (2) complete seperate AH game installs and even my AH backup folder on my WinXP install of my dualboot system.....
to keep it short.... I never realized what I did until I went to logon for FSO friday night..... thank goodness I had Windows 7 installed on the same HD set in dualboot mode.. I was able to reboot and logon to Win7 and play FSO friday night.... ( actually I have 3 HD's and can boot to 4 different installed OS's depending on what I am wanting to do, along with an extra "hotswap HD set up" as well )
and I even have most of my backup files I can copy back over to my WinXP OS after I re-install the game..... ( lost nearly 7 gigs worth of stuff with a very simple mis-click of the mouse.... on that particular partition )
if you going to have at least 4 gigs of memory & a dual core CPU....... you will really benefit with windows 7 64 bit..... even if though most everything the majority uses is 32 bit oriented