Open source software

With the help of Greg Neumarke (ACM, Chicago)

WEB BROWSERS
Google's Chrome web browser
probably the safest browser, fast, and automatically keeps the ever-vulnerable Adobe flash player updated.
Best to install it using the network installer here:
http://www.google.com/chrome/intl/en/landing_chrome.html?hl=en

Firefox 4.0 Web browser
is another good option
http://www.mozilla.com/en-US/firefox/new/

Adobe Flash Player
Flash is needed on a lot of web sites, and is frequently updated to more secure versions. The latest one for IE and Firefox is version 10.2.153.1
Download the IE (ActiveX) and Firefox manual installers here:
http://kb2.adobe.com/cps/191/tn_19166.html#main_ManualInstaller

PDF
PDF-xchange-viewer
Recently found this program, it reads PDFs, but also lets you annotate them, so it's good for filling out forms, for example. Smaller and faster than the huge Adobe Reader program.
http://www.tracker-software.com/product/pdf-xchange-viewer

PDFCreator
I install this on almost every computer I work on. It installs a "virtual printer" that shows up in your list of printers. When you print to it from any program, it creates a PDF file of whatever you were printing. Good for systems with no real printer. You can then transfer your PDF to a system that has a printer, or just save the PDF.
http://sourceforge.net/projects/pdfcreator/

FILE COMPRESSION
PeaZip
This program will open or create any kind of compressed file you run into. Similar to WinZip, but free. Just be careful when installing it to deselect the optional toolbar.
http://www.peazip.org/

CD/DVD
CDBurnerXP
If the system has a CD or DVD Burner, but no software for using it, this is a good program for a Windows XP machine. (I haven't had as much luck with in on Windows 7)
http://www.cdburnerxp.se/

ImgBurn
If you just need to burn ISO files to CDs or DVDs, this is a simple one-purpose program. (Good for say, burning an Ubuntu ISO file to CD)
http://www.imgburn.com/

IMAGE MANIPULATION
Picasa, from Google
This is an excellent program for managing digital photos, and doing simple cropping and editing.
http://picasa.google.com/

Paint.net
Painting / Drawing / Photo program similar to Photoshop. Built using .net framework. Note that it requires .net 3.5 SP1 on Window XP SP3 as a minimum
http://www.getpaint.net/index.html
Dot Net Framework 4 installer: http://www.microsoft.com/downloads/en/details.aspx?displaylang=en&FamilyID=0a391abd-25c1-4fc0-919f-b21f31ab88b7

Dia
A flowcharting, diagramming program sort of like Visio
http://dia-installer.de/index.html.en

Inkscape
A vector-graphics based drawing program
http://inkscape.org/download/?lang=en

MEDIA PLAYER
VLC VideoLan Player
This program can play pretty much any sound or video file that exists.
http://www.videolan.org/

OFFICE SUITE
LibreOffice 3.3.2
This is the better and newer version of what used to be called OpenOffice.org. Contains a word processor, spreadsheet, presentation program, and more. Compatible with reading and saving Microsoft Office file formats.
Download both the main program installer (which is multi-language) and the separate help packs in whatever languages you might need.
http://www.libreoffice.org/

EDUTAINMENT
Tux Paint
A children's drawing program with many options, sound, and an add-on pack of image stamps. Download the main program and the "stamps" add-on
http://www.tuxpaint.org/

Childsplay
A variety of educational games in one program. Click through to the "childsplay" section to get to the download. Supports different languages.
http://www.schoolsplay.org/

ANTIVIRUS  / ANTISPYWARE
Computers in Africa are infamous for being infected with viruses. I would use your flash drive sparingly, and load software from CDs. (You are likley to encounter infected computers that will try to write viruses to your flash drive as soon as you put it in)

Microsoft Security Essentials
Free antivirus program from microsoft. Simple, quiet, and effective. Of course, needs internet access to update definitions.
http://www.microsoft.com/en-us/security_essentials/default.aspx

McAfee Stinger
A small program that doesn't need internet access to run. It checks for many common viruses.
http://www.mcafee.com/us/downloads/free-tools/how-to-use-stinger.aspx

Microsoft Windows Malicious Software Removal Tool (MSRT)
Another standalone program that scans for a subset of common viruses, doesn't need internet access.
http://www.microsoft.com/downloads/en/details.aspx?FamilyId=AD724AE0-E72D-4F54-9AB3-75B8EB148356&displaylang=en

SuperAntiSpyware
An antispyware program. You can download a standalone portable version that doesn't need internet access
http://www.superantispyware.com/portablescanner.html?tag=SAS_HOMEPAGE

Spybot Search & Destroy
Anti-spyware program that has been around a long time. Download the main program, version 1.6.2, and the "detection updates" so you  can have updated definitions without internet access. I like this one because it can give you more information about what it finds.
http://www.safer-networking.org/en/download/index.html

LINUX
Ubuntu Live CD
I would download and burn a CD copy of the current version of Ubuntu 10.10 Desktop edition so you can boot a computer and use it, get online, etc, without worrying about viruses.
http://www.ubuntu.com/

WINDOWS XP service packs
Only Service Pack 3 is supported by Microsoft anymore, so consider bringing SP2 and SP3 if you run into computers that aren't updated to XP Service Pack 3 yet.
You'll need SP2 and SP3 (As it is recommended to do SP2 first if the computer is at SP1 or lower)
Note that these are pretty large, although both SP2 and SP3 would fit on a single CD.
Windows XP Service Pack 2 (SP2)
http://www.microsoft.com/downloads/en/details.aspx?FamilyID=049c9dbe-3b8e-4f30-8245-9e368d3cdb5a

Windows XP Service Pack 3 (SP3)
http://www.microsoft.com/downloads/en/details.aspx?FamilyId=5B33B5A8-5E76-401F-BE08-1E1555D4F3D4&displaylang=en