This is my list of essential or at least useful, free (for personal use) programs for Windows.
AntiVirus
AntiVir This is the one that I've been using. It seems fairly light and quick (now thinking about switching to AVG as AntiVir throws up a huge advert whenever it updates).
AVG Free has been around for a long time. It's regularly updated and is generally unobtrusive.
avast! 4 Home Edition has also been around a while but I haven't used it in a long time.
Kaspersky is reputed to be one of the best Antivirus programs/suites out there. They have a free online scanner and a free 30 day trial
These are mostly down to personal preference.
Anti Spam
SpamBayes Excellent Outlook addin, which when fully trained can be 99.9% efficient.
Anti Spyware
Spybot Search and Destroy Very good anti spyware program, can be a little over protective if you install all components.
Hijackthis Seems that HijackThis has been taken over by TrendMicro, which can be no bad thing. This is a fairly complex program and it's recommended that you post your HijackThis log in the forums for an expert to check. This is the best piece of software for removing those annoying pieces of spyware which repeatedly reset your browsers home page and spam you with pop ups.
Windows Defender Antispyware from Microsoft. Comes bundled with Windows Vista, but can be downloaded for free for Windows XP.
Malware Bytes this program has been useful in removing some of the fake Antivirus viruses/spyware that's been going around lately.
Archiving
Windows has had a built in archiving program for .zip files since Windows XP (making winzip pretty much redundant). Simply double click a .zip file to open it and to create one right click a file/folder and choose Send to > Compressed (zip) folder. However there are a few more archiving formats out there, which whilst not as popular as .zip are often far superior in their compressing ability.
7zip is the best totally free archiving program I've found (even for business use).
Supported formats:
Packing / unpacking: 7z, ZIP, GZIP, BZIP2 and TAR
Unpacking only: RAR, CAB, ISO, ARJ, LZH, CHM, MSI, WIM, Z, CPIO, RPM, DEB and NSIS
I used to use programs like WinAce and WinRar but these were only trials and had a more bulky interface and seemed to lack the speed of 7zip. The only downside of 7zip is not being able to create .rar files.
Browsers
Opera The fastest browser out there. Also available for mobile devices.
Firefox Probably the second most popular browser after Internet Explorer, due to it's many community made addins.
Internet Explorer 7 Microsoft has now made it's latest version of Internet Explorer available for everyone. If you're running on Windows XP, you can now upgrade to IE7.
Internet Explorer 8 Beta The next version of Microsoft's browser shows a few neat additions, be careful if you choose to install this though, as it will replace your current version of Internet Explorer and it's still only in Beta stage.
Google Chrome Beta The latest offering from Google, this has a lot of good ideas and is well worth a try, it's not quite ready to be your only browser yet though as it's also still in Beta stage. Looking forward to see what's next from Google, perhaps an OS?
CD/DVD Burning
ImgBurn ISO creation and burning. A simple little program, which has some very advanced features should you need them.
Daemon Tools Handy program that allows you to mount images and present them as if they were a real disc in a drive.
Communication
Windows Live Messenger Windows Live messenger, the latest incarnation of MSN messenger. Practically everyone uses this.
Skype Very similar to MSN in that you can do instant messaging and voice chat. Skype is promoted more as a VOIP solution and allows you to make calls to real world numbers. You can also have a Skype number so that people can call you from real world phones (at a cost of course). It's also possible to get Skype phones, which can connect to a wireless internet connection and then log you into your Skype account.
Disk Management
Diskeeper Lite Every PC (apart from Vista apparently) could do with a good defrag to keep file access as quick as possible. Hard drives are typically the slowest part of any system so keeping them as quick as possible really does make a difference.
This is the built in windows defragger just with the full user interface. As such if you install this program it will replace certain key files to do with the built in defragger, if you then decide to uninstall this, it will take the key files with it and you will need to fix the built in defragger to get it to work again.
Media Players
vlc This is a great player, which can play virtually any file, including quick time (so you don't need to install quick time). This is an essential program, you really don't need anything else for playing files.
According to the features list the only files it can't play are real player files. I'd recommend avoiding these files if possible as real player is awful.
Picture Management/Editing
Google Picasa Great for picture management and can do basic enhancement.
paint.net A powerful but simple to use editor.
The Gimp More powerful than paint.net but more complicated.
Windows Tweaking
TweakUI - XP Part of the Microsoft powertoys. TweakUI allows you to alter many hidden parameters of Windows XP.
TweakVI - Vista This is the Vista equivalent of TweakUI, but isn't from Microsoft.It is very good however.
More
Get Acrobat Reader
Get Flash
Get Java
Google Sketch Up
OpenOffice
Dev C++
Python