Not designed =/= Not possible. Windows 3.0 and 3.1 are both officially supported and well documented. Not sure about Windows 95 or 98 -- possible but not supported? Perhaps somebody has a copy of the Resource Kit 3" thick book handy? [Edit: 3Com have some info on how to do it.]
Windows NT 4, 2000 and XP are network bootable using third party software. The PC NIC has to be RIS compliant IIRC and some companies sold boot ROMs for popular NIC adapters. The software was offered to the education and similar markets in Europe several years ago, although it never made much sense to me on an economic basis (network hardware costs more than hard disks). I've also seen remote boot software for PC assemblers that that uses an XP rather than Win PE image for hardware testing.