Windows Firewall, as far as I recall, doesn't do much (if any) control over outgoings, while the router does a pretty good job of stopping incomings - stops 'em all, unless you tell it to make holes for things.
There are quite a few free ones that offer more control than the Windows firewall, as behind a router, the software firewall has a role which is more administrative that protective - well, stopping unwanted connect-outs that may indicate undisclosed adware/spyware.
http://www.firewallleaktester.com/tests.php - Jetico free came top in this leaktest, doing best at stopping the various surreptitious ways that things can try to piggyback on allowed connections, though you are only likely to be "leaked" if you have already been infected by leaky malware - lets say some trojan has got in, got your credit card number, and wants to send it back to its master...
The "classic" leak would be to co-opt Internet explorer in some way, and then access the fraudster's website with the information in a ? parameter. The DNS method, would be to direct a DNS request to the fraudster's server, with the name of the site requsted being the information ... the "basic" protection in many firewalls for this is a bar on long/malformed DNS requests - the complex way is user specified DNS rules, so only the legitimate DNS server can be used.
Have I scared you yet?
Seriously, all this requires that you are ALREADY COMPROMISED, and could be considered to be locking the stable door while the horse is trying to bolt through it.
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