Check if you need the 32 or 64 bit version (Above link is for 32bit version)

FireFox

The Best alternative Web Browser to Internet Explorer for XP and Vista Now at Version 6 or a beta version 7 is available

Outlook Express
Free popular Email program for Win98-XP Vista has windows mail already.
Windows 7 can use windows Live Mail 2011 (Part of Windows Live Essentials 2011)

Adobe Acrobat Reader
Free PDF document reader/viewer now at version 10.1 for XP/Vista/7 (older versions are compatible with win 98-ME)

WS-FTP Home
FTP upload program - for publishing files to the internet.

Winzip

File Compression Utility for unzipping (expanding) .zip and more files Not free for ever - Shareware

Google Picasa 3
No question the best free digital picture image manager/viewer/editor for non-commercial use.

Superantisyware

A Superb free tool for Virus and Spyware removal

Microsoft Security Essentials If you load only one security tool on your PC make it this one - FREE from Microsoft.

Malwarebytes

Malwarebytes Anti Spy/root/virus/trojan/malware Cleaner Tool.  I have used this for a couple of years- one of the very best free tools available

AVG Anti Virus

The best of the Free Anti Virus Program Noticeably less overhead than Kaspersky, McAfee or Norton.  From Grisoft now at version 10 recommended if you have a powerful modern PC for older units use Microsoft Security Essentials above.