Open Source Software for Windows:

OpenOffice
OpenOffice.org is both an open-source application and project.
It is free. The product is a multi-platform office productivity
suite compatible with all major file formats.

ClamWin Antivirus
ClamWin Antivirus is an Open Source (free) Virus Scanner Toolkit
for Microsoft Windows NT/98/Me/2000/XP/2003. It provides
User Interface to Clam AntiVirus scanning engine.

Mozilla
For web browsing, email, HTML editing, IRC chat, and more.
Built with your privacy and security in mind. Stop popups and
junk mail. Open multiple web pages in the same window with
tabbed browsing.

Mozilla Thunderbird
A cross platform stand alone mail application using the XUL user
interface language. In addition to the feature set found in
Mozilla Mail, Mozilla Thunderbird has several new features and
improvements to make your mail and new experience better.

PDFCreator
PDFCreator is a free tool to create PDFs easily from nearly
any application. With the PDFCreator Printer driver you turn
any program into a PDF-machine.

Filezilla
FileZilla is a fast and reliable FTP client and server with lots of
useful features and an intuitive interface.

The GIMP
The GIMP is the GNU Image Manipulation Program. It is a
freely distributed piece of software for such tasks as photo
retouching, image composition and image authoring. It works
on many operating systems, in many languages.

VideoLan
Multimedia streaming of MPEG-1, MPEG-2, MPEG-4 and DivX files,
DVDs, digital satellite channels, digital terrestial television channels
and live videos on a high-bandwidth IPv4 or IPv6 network in unicast
or multicast under many OSes. VideoLAN also features a
cross-plaform multimedia player, VLC, which can be used to read
the stream from the network or display video read locally on the
computer under all GNU/Linux flavours, all BSD flavours, Windows,
Mac OS X, BeOS, Solaris, QNX, Familiar Linux...

TurboCASH .7 Open Source Accounting
Powerful accounting software that's flexible. Turbocash features
everything one would expect from accounting software costing
£250 or more. And since it's governed by the GPL, TurboCASH is
free - no 30-day trials or restricted versions.

SharpDevelop
#develop is a free IDE for C# and VB.NET projects on
Microsoft's .NET platform. It is open-source (GPL), and you
can download both sourcecode and executables from this site.

TrueCrypt
Free open-source disk encryption for Windows XP/2000/2003

DIA
Dia is a diagram drawing software similar to Visio.

Open Workbench
The Open Source alternative to MS Project. Open Workbench
is a stand-alone desktop application that provides robust
project scheduling functionality.