Despite being free, these packages are not in the public domain and each comes with license conditions.

Software for All Users:

Web Browser

The browser I use everyday is Firefox which I use on Windows and Linux. I also like Opera.

E-mail Client

An e-mail client is a fairly simple program and there are many good ones (even Microsoft's Outlook Express is minimally acceptable). I now use Thunderbird except at work where they won't let me. Its spam filter is very good.

Office Suite

Rather than pay through the nose for Microsoft Office, I use Open Office which provides replacements for Microsoft Word, Excel, Powerpoint and Access. I also use it on Linux.

Anti-Virus

I find AVG Anti-Virus Free Edition to be better than famous-name anti-spyware that charges a yearly subscription. AVG's automated updating is always error-free.

Anti-Spyware

I use both Spobot Search&Destroy and Ad-Aware. Recent tests have shown that spyware products don't do a very thorough job so it's worth using a couple of different products.

Firewall

A hardware-based firewall (e.g., in a router) is preferable to a software-based one because software-based firewalls sometimes cause mysterious problems with Windows applications. If you must use a software-based firewall, I used ZoneAlarm for a couple of years with few problems. Most people will be satisfied with the free personal version.

File Erasure

When you delete a file, its contents remain on the hard disk. Fairly simple tools can recover this data. Sensitive files should be erased by a program specially written for this purpose. Eraser is easy to use and will ensure that a scientist with an electron microscope will be unable to recover your deleted files.

Clock Accuracy

If you want your system clock to be accurate, Atomic Clock Sync can do this either automatically or whenever you run it manually. It is a very easy program to install and use.

Password Database

I have more user-ids and passwords than I can remember. So I store them in a KeePass database which uses strong encryption and is controlled by a single master password.

Portable Document Format (PDF) Reader

Adobe's free Acrobat Reader is large and slow. I've been much happier with Foxit Reader for Windows.

Compression

Rather than pay for PKZIP or WinZip, you can use 7-Zip which supports several compression algorithms and includes strong encryption.

Disk Encryption

TrueCrypt can encrypt an entire disk partition or USB drive, and can create a virtual encrypted disk within a file.

Entertainment

I'm not a computer game player but I do occasionally play chess against GNU Chess for Windows.

Software for Programmers:

Programming Language

Unlike Unix/Linux systems, Windows does not ship with many useful programming languages. For scripting and general-purpose programming, I like Python. It's an interpreted, object-oriented language with simple syntax and a large collection of standard libraries. It's elegant and reliable, and is simple enough for a beginner but rich enough for a pro. I use Active State's ActivePython on Windows.

Text Editor

Notepad and WordPad, the text editors that are shipped with Windows, have almost no features. There are many excellent commercial text editors (I like TextPad) but Notepad2 or, if you want more features, Notepad++ are just as good.