Actually Win CE can run as low as 1 MB for the kernel alone.
The gizmondo ran it, with only marginally better specs than the nspire.
http://en.wikipedia.org/wiki/GizmondoKeep in mind there is no ARM version of win 95/98/XP/etc. The only ARM windows that would be suitable is CE, outside emulation.
If you really want emulation, keep in mind the ~6x performance penalty. That would reduce the 150 on the nspire to less than the 33 MHz standard used by most DOS computers.