What do you use now?
See post #5. 1 thing Microsoft AntiSpyware (Beta1) requires Windows XP.
Here is the exact OS requirements:
Supported Operating Systems: Windows 2000, Windows 2000 Advanced Server, Windows 2000 Professional Edition , Windows 2000 Server, Windows 2000 Service Pack 2, Windows 2000 Service Pack 3, Windows 2000 Service Pack 4, Windows Server 2003, Windows XP, Windows XP Home Edition , Windows XP Media Center Edition, Windows XP Professional Edition , Windows XP Service Pack 1, Windows XP Service Pack 2, Windows XP Tablet PC Edition