i have 2 HDDs with RedHat 7.2 and Win98... i boot using GRUB and i like it better than LILO. however, if you install linux first, the windows installation will over-write the boot sector and GRUB will not run. so,
you should install linux second so it will control the boot process.
however, when i had Win2000, Win98 and RedHat7.0 on the same machine a couple years ago, i remember the Win2k saying something about having to install it LAST. i bet XP is the same way, and therefore your nephew will have to resort to booting linux with a floppy.
disclaimer: i am a linux newbie, so do not believe anything i tell you.