Migrating won't be easy, but depending on your requirements, can substantially reduce licensing costs.
Most Linux distributions use the Gnu General Public License. This document (30-page PDF) is a comparison of the Windows EULA and the GPL. In general, the GPL generally bestows user rights where the EULA specifies user restrictions and indemnifies Microsoft. I don't know what the MacOS user license looks like..
As to migrating from Windows to a Linux distribution, your requirements will determine whether that is possible and how easy or hard it will be. Right up front, AutoCAD, Solidworks, and Photoshop do not have Linux/Unix versions, although alternatives are available. If you use these applications in your business, complete migration may not be possible; you may have to stay with Windows for the workstations using those applications. If your requirements are for basic office software (office suite, email, internet), then the change to a Linux distro may be fairly easy, requiring only some user training.
I don't know what your requirements are, so I can't recommend a particular Linux distribution to you. Each distrubution is different and each is built for a purpose. My suggestion is that you review the choices and options, download some distributions, install them, and use them for a while to evaluate how well they will meet your requirements. I recommend Ubuntu, PCLinuxOS, and Mandriva as good starting points. You can find links to most Linux distributions at
http://distrowatch.com/.
Other Linux-related links:
Migration tips -
http://www.linuxlinks.com/beginners/Linux "home page" -
http://www.linux.org/Migration ebook (free) -
http://happi2share.blogspot.com/2009/05/moving-from-windows-to-linux.htmlMore migration tips -
http://www.computerworld.com/action/article.do?command=viewArticleBasic&articleId=9033804Linux equivalents of Windows applications -
http://www.linuxlinks.com/article/20070701111340544/Equivalents.htmlEven more migration tips -
http://www.ibm.com/developerworks/linux/library/l-roadmap.html 
Moderator 
