I think this has been covered several times in other answers posts. Netflow over-the-wire is not a textual format. It is a very dense binary format, and needs the assistance of some tools to format the netflow binary data into something textual (and therefore consumable by Splunk).
There is an existing app in SplunkBase, http://splunkbase.splunk.com/apps/All/4.x/app:Splunk+for+NetFlow that has some of the necessary plumbing in place. It depends on the nfdump tool from http://nfdump.sourceforge.net/ . Whether or not nfdump works on windows is probably going to be your biggest hurdle to manage.
Some other related answers:
http://answers.splunk.com/questions/10242/how-to-configure-nfdump-to-convert-netflow-data-from-binary-to-text-or-csv
http://answers.splunk.com/questions/8162/how-can-i-use-netflow-in-splunk-when-its-on-a-windows-box-without-buying-more-so
http://answers.splunk.com/questions/4186/how-can-i-index-netflow
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