Whilst I'm in the process of adding a signifier to the freeware available on my website to show it'll work under XP, here's an interim list of some of those found so far :

<font color=448800>XP Compatible Freeware</font color=448800>

AdAware 5.6
Zone Alarm 2.6.357
Check Links
Quick Tray
IE New Window Maximizer
RealPlayer 8 Basic
AirBoard Manager
MS Intellipoint
NoAds 2000.6.30.1
PaintShop Pro 5
Adobe Acrobat Reader 5
Add Remove Pro v2.04
CDex 1.4 beta 8
Download Express 1.0
Power Desk 4
Irfan View 3.51
Google Search Bar ( it's IE6)
Smart Board 32 (retains its content across any platform)
Easy Clean Reg Cleaner
Disk Space
Hard Copy 14.2.01
Norton Secret (simple encryption)
Free Shade (rolls windows up)
Ghost Script (PDF maker)
HTML Color Calculator
Wilbur File Indexer
KillProcess (app killer)
Patch Verifier (Steve Gibson)
Pic View (Slide Shower)
UltraFunk Popcorn (Mail server)
RenameIt 3.1.1.c
CKRename 1.08
XTeQ Tweaker
SiSoft Sandra
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Those on my pages with a Windows flag work correctly under XP, those with the red circle do not, and those with no identifier haven't been tested yet.