The easiest way is installing it last. Like if you wanna dual boot Win2K and Win98, you install Win98 first then Win2K, right? If you want Linux in there, it is more 'aware' of other OS's then even 2K (as opposed to the dumb and blind Win9x line), and will take care of things. Just put/move your other MS OS's on there to have a good 2 or 3 gigs free. I've done Mandrakw isntalls that took about 3 gigs, but most of my Redhat installs seem to take only 1.5. I've heard that Mandrake is more bloated, but the difference might be me being lazy when choosing packages. Anyway, you install Linux into some free space (and last), and install Lilo into the MBR. It'll figure things out and have a "win" or "windows" entry by default, which will load up the NT boot menu.
That's the easiest to boot and isntall. The easiest, just as far as installing goes, is to not install Lilo and use a boot disk for Linux.