Follow below Steps :
modify the Oraparam.ini file located here
"database\install\oraparam.ini"
[Certified Versions]
# You can customise error message shown for failure, provide value for
CERTIFIED_VERSION_FAILURE_MESSAGE
Windows = 5.0,5.1,5.2,6.0,6.1
[Windows-6.1-required]
# Minimum display colours for OUI to run
MIN_DISPLAY_COLORS = 256
# Minimum CPU speed required for OUI
# CPU = 300
Save the oraparam.ini file & then run setup again. Now all goes fine & got the oracle installed.
There is one more way i will found it here. if above mention solution will not work then check this link
http://doganay.wordpress.com/2011/01/31/checking-operating-system-version-must-be-5-0-5-1-5-2-or-6-0-actual-6-1-failed/
Try this solution if not work check the link and then let me know