Microsoft has released Windows Messenger version 4.5 for Windows XP.

Minimum System Requirements
Computer/Processor : Computer with 300 megahertz or higher processor clock speed recommended; 233 MHz minimum required (single or dual processor system);* Intel Pentium/Celeron family, or AMD K6/Athlon/Duron family, or compatible processor recommended
Operating System : Windows XP Professional or Windows XP Home Edition
Disk Space: 10 MB available for installation
Memory: 128 MB of RAM or higher recommended (64MB minimum supported; may limit performance and some features)

Video Display Super VGA (800 X 600) or higher resolution video adapter and monitor

Messenger 4.5 seems to work fine also under Windows 2000 Professional

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