Window Lock Suggestions ?

Hi,
Moved into a house that has several wooden windows (the normal up-down kind) at pretty close to ground level Semi-rural setting.
Would like to add some decent window locks, but am very uncertain what I want to get. Would sure appreciate any opinions.
First thought is that as these windows do provide an emergency fire egress possibilty, so I don't want any that have an actual keyed lock to operate.
Also, and here's the problem I guess, I would like a type that would be very hard for anyone willing to break the glass and reach inside to operate the catch and therefore gain entrance.
Still, we would like to be able to disable reasonably easily so we could, on occasion, open the window if we wish.
Are there any styles or mfg's that approximately meet these requirements, and that you can recommend ?
Thanks, B.
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Sometimes the best solutions are the simplest.
Drill a hole in the lower sash and 1/2" into the upper sash where the two sashes meet. Insert a large nail, carriage bolt or old hinge pin into the hole to pin the sashes together and prevent either from operating. The lock is invisible from the outside but easily removed from the inside if you don't fit it too tight.
If you insist on a "lock", http://www.blainewindow.com .
John
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