installation interrupted

Whenever I click to install the program, a window pops up
with the message "Microsoft AntiSpyware requires that your
computer is running Windows 2000 or Windows XP or Windows
2003 Server." After I click 'OK' from that window, another
screen comes up and says "The wizard was interrupted before
Microsoft AntiSpyware could be completely installed. Click
Finish to exit the wizard." This is keeping me from
downloading the program. Why is this happening??? Please
help...I need this software!