We often get questions about how to provide a Unix environment under Windows. Mircosoft has several OS's that collectively are called Windows. Most users are running a recent Microsoft OS with an NT kernel, usually Windows XP. The solutions in this thread may not work on very old Microsoft OS's. There are three basic packages that provide a unix-like environment under Microsoft Windows: SFU, Cygwin, and uwin. There is also a relatively new player called colinux. A new product is available from VMWARE for freecalled VMware Server. Also when this question comes up, users sometimes suggest using vmware, or to run Linux from a cd, or to partition the hard drive and install linux or SunOS. I'll include links to threads that mention these ideas.

SFU
http://www.microsoft.com/windows/sfu/
Microsoft Services For Unix used to be a product, but as of this writing, it's a free download. It comes with pdksh and tcsh. Some more packages for sfu are available here including bash. There is a SFU FAQ that you should probably should read before installing sfu.

Cygwin
http://cygwin.com/
Cygwin is also free. It comes with bash and many other gnu packages. It uses bash. There is a Cygwin FAQ with extensive information.

uwin
http://www.research.att.com/sw/tools/uwin/
uwin was written by ATT and it also is free. Formerly, uwin plus some extensions were sold as a product by Wipro under the name Wipro Uwin. Wipro discontinued the uwin product as of May 7, 2004. There is a uwin FAQ

colinux
http://www.colinux.org/
Cooperative Linux is the first working free and open source method for optimally running Linux on Microsoft Windows natively. More generally, Cooperative Linux (short-named coLinux) is a port of the Linux kernel that allows it to run cooperatively alongside another operating system on a single machine. Here is the colinux FAQ.

VMware Server
http://www.vmware.com/products/server/
VMware Server is a product for server virtualization. VMware Server enables you to partition a physical server into multiple virtual machines. You can run Windows and Linux operating systems and applications on the same physical server, at the same time. Here is the VMware Server FAQ

THREADS
Here are some threads that discuss all of the options:

UNIX for Windows?
UNIX emulators
Questions about UWIN?
Windows Services for UNIX
need help with cygwin installation
From WinXP to Unix with X-window
free VMWARE clone for windows?
about cygwin
Windows UNIX emulators?
UNIX in MS Win2003 Domain
what is the use of cat32 command
windows -linux dual boot............