OpenOffice - free office suite, replaces Microsoft Office, almost entirely compatible with Word and Excel, also prints PDFs
Google Earth - photos of anywhere in the world, needs broadband connection
Trillian - combines MSN Messenger, Yahoo! Messenger and AIM in one program
VLC Media Player - free DVD playing software, not region-coded, great all-round media player
Tweak UI Power Toy - lets you fiddle with hidden XP settings
Ad Aware Personal - same as Spybot, I run both anti spyware programs together
Windows Defender - ditto, free anti spyware software from Microsoft
K Lite Codec Pack - decoders for all movie & music file types, plus Real Alternative and Media Player Classic to replace bloated XP software
And if you download Firefox (listed above) there are some excellent add-ons for it:
Ad Block Plus - filters banners and Flash ads, even Google text ads, from web pages
No Script - blocks all scripts and executable content from web pages unless you allow them
IE Tab - runs IE in a Firefox page, so you can open pages that don't like Firefox
Mouse Gestures - navigate the web in Firefox using combinations of mouse movements