I believe that the single biggest reason that people are reluctant to switch to an open-source OS is the fact that they are unfamiliar with the software or what programs offer an equivalent linux version of the programs they are used to using with their windows box. The truth is their is far more software available for Linux systems than there are for Windows. The same however can not be said for Apple's OSX, traditionally it has been hard to find software for Macs. Their are a few cross-platform apps out there that I use on all my systems no matter what OS:
Firefox Web Browser
Adobe Acrobat PDF
and there are some open-source packages that offer more than adequate substitute for there windows counter parts:
Open office does everything that the MS office suite will do and more and it even looks identical. Not to mention it is cross platform and free. This covers Word, Excel, Powerpoint, and Access.
Kompozer is a fine substitute for Frontpage or Expressions.
Scribus works in place of Publisher.
Gimp works for Photoshop.
Evolution mail is a great mail client that works with almost any protocol or server system.
Amorok and movie player provide a great replacement for media player, and Rhythmbox is a iTunes clone.
Kmymoney or Gnucash provide alternatives to MS Money or Qucken.
Here is a site that provides a much more complete list of apps note that the open-source alternatives are almost 2 to one over popular windows apps and a great deal of them are cross platform. http://wiki.linuxquestions.org/wiki/Linux_software_equivalent_to_Windows_software
If that's not enough for you check out the WINE project (Wine Is Not an Emmulator) it allows you to install and run windows software in a Linux OS environment.