SatCom and Computers can provide commercial open source solutions for your business.  Some of these solutions includes: file server (Store all of your files on centralized server for ease of sharing and backup), email server (bring your email in house for enhanced security and performance), groupware server (share calendars, documents, etc. throughout your company.)  You can have all of the features of Microsoft Server products, Sharepoint and Exchange at a fraction of the cost.

Why Linux?

Price

Linux distributions are definately less expensive than it's Windows counterparts.  From $0 - $300 for an Enterprise Linux version compared to $100 -$1,500 for it's Windows counterparts, you can see the savings.  Linux was derived as an Open Source product and its different distributions are constantly being updated by individuals and companies that contribute to this endeavor.  Typically, Linux companies don't charge for the software or distributions, but they charge for a higher level of tech support. 

Stability

Being a Windows user, I'm very accustomed to rebooting my computer.  Windows computers, whether desktop or server, often need to be rebooted.  Linux is practically imune to this problem.  Many Linux desktops go weeks and weeks without a reboot, many Linux servers go months and years between reboots. 

Also, Windows machines, especially XP can be very susceptible to slowness caused by hard drive fragmentation or file corruption due to lost chains or bad hard drive sectors.  Linux has a much more stable file system and never needs any defragmentation on the hard drive.

Here is a comparison from http://www.whylinuxisbetter.net.

If you already know what fragmentation is, and are already used to defragmenting your disk every month or so, here is the short version : Linux doesn't need defragmenting. Now imagine your hard disk is a huge file cabinet, with millions of drawers. Each drawer can only contain a fixed amount of data. Therefore, files that are larger than what such a drawer can contain need to be split up. Some files are so large that they need thousands of drawers. And of course, accessing these files is much easier when the drawers they occupy are close to one another in the file cabinet.
Now imagine you're the owner of this file cabinet, but you don't have time to take care of it, and you want to hire someone to take care of it for you. Two people come for the job, a woman and a man.

• The man has the following strategy : he just empties the drawers when a file is removed, splits up any new file into smaller pieces the size of a drawer, and randomly stuffs each piece into the first available empty drawer. When you mention that this makes it rather difficult to find all the pieces of a particular file, the response is that a dozen boys must be hired every weekend to put the chest back in order.
• The woman has a different technique : she keeps track, on a piece of paper, of contiguous empty drawers. When a new file arrives, she searches this list for a sufficiently long row of empty drawers, and this is where the file is placed. In this way, provided there is enough activity, the file cabinet is always tidy.

Without a doubt, you should hire the woman (you should have known it, women are much better organized :) ). Well, Windows uses the first method ; Linux uses the second one. The more you use Windows, the slower it is to access files ; the more you use Linux, the faster it is. The choice is up to you!

Ubuntu.  Linux with Support

Those are two words that do not usually go together.  Most Linux distributions usually provide very little support.  (Mostly in the form of forums and mailing list.).  Ubuntu has changed this; they provide both free community support via forums and paid professional support.  I have found Ubuntu to be a very stable replacement for Windows server applications and have personally run web, email, and file servers on it. 

Zimbra Open Source Collaboration Suite

Zimbra provides an affordable solution to providing group email, calendars, and document sharing for your entire business operations.  Zimbra can work standalone or coexist with Microsoft products like Active Directory and Outlook.  It has an AJAX web client that allows the user access to all of the features that you would normally see in Microsoft Outlook in a thin client design.  So no matter how many employees you have, there are no client side licensing fees.  You pay for the licenses on the server, which covers your support, and that's it.