Have a look at UWIN
http://www.research.att.com/sw/tools/uwin/
(internal development, now on Common Public License, is almost UNIX but they don't care about certification)
further,
WinXP and lower can download SFU for free from MSFT,
Server 2003, Vista Business|Premium have the SUA, just need to be enabled (both bsd44 based).
No need to dual boot a laptop to get both worlds.
Need Linux, have a look at coLinux: any Linux.
So, you boot Windows? Big deal
