I've heard you can drill a hole in the window frame up where the lock is and put a penny nail in. It's suppose to works best if the hole is drilled at a angle with the pointof the nail when placed in the hole faces towards the ground.
I've no idea how well it works.
There are also alarms available for spaces like this that are battery operated. The sound alone may frighten off an intruder.
If your shrubs are over grown trim them so your would be intruder doesn't have a place to hide.
If you don't have shrubs under your windows you may want to think about planting some that are thorny or prickly...like a holly.

You can buy secondary locks at any hardware place.

Do not nail your windows shut as you need an escape for emergency like fire.

Here is an inexpensive way to secure windows: Drill a hole in the upper window portion about three inches above the split. Then *insert* a large nail so that the lower window proation will hit the nail if it is opened. This way you can have the window open three inches for a breeze if you wish and still prevent someone from the outside from opening it further. Drill the hole slightly larger than the nail so it can be inserted and pulled out with ease. You need to do it this way so that you can open the window all the way if you need to use it as an exit but quickly pulling the big nail out.

I agree with bud. We've done that in our homes and it works well.

I saw these on QVC. They are called BurglaBar 6-piece Silent Entry Security Set Item # V22466
They are for sliding glass doors as well as doublehung windows. You get a set of 6. Basically, a BurglaBar is a small rectangle of incredibly strong clear lucite plastic. It adheres to your window (or sliding glass door). It has a part that flips up or down. In the down position you can open your window. In the up position it blocks the window from moving. You can even place them on the window so that you can open your window only a few inches but no more than that (for people who want to have a little fresh air but don't want anyone to enter thru the window).

It is hard to explain what they are and the photos on QVC. com don't really help, but you can go to QVC.com and read about the product. Read the reviews of it too. If you buy something from QVC and don't like it, you can send it back for a full refund within 30 days, no questions asked.

I have done the drilling a hole for a nail thing too, as other posters have suggested. It works but is not very attractive. I even painted the nails white to match the window frame to try to improve the appearance of it. What I found was that you have to align both sashes perfectly while you are drilling the hole. SO if your sashes are kind of old and crooked like mine, have someone tightly hold the window closed while you are drilling.

Thank you all for your responses. I had heard of the installing a nail and I am glad you all pointed out the need to have it easily removed (didn't think of that).

carol_from_ny - thanks for your tips on the shrubbery. Planting roses under windows....hmmm...I am always looking for more areas to plant roses! I will also check out those battery alarms. They sound very convenient.

Bud_wi ..... your method involves installing the nail on the face of the sash, correct? (not on the inside edge of sash where wood meets glass). What size nails do you use?

Buddyben .... these look interesting. I like the fact that they appear to be a "aesthetic" solution too.

Thanks everyone!

Thank you all for your responses. I had heard of the installing a nail and I am glad you all pointed out the need to have it easily removed (didn't think of that).

carol_from_ny - thanks for your tips on the shrubbery. Planting roses under windows....hmmm...I am always looking for more areas to plant roses! I will also check out those battery alarms. They sound very convenient.

Bud_wi ..... your method involves installing the nail on the face of the sash, correct? (not on the inside edge of sash where wood meets glass). What size nails do you use?

Buddyben .... these look interesting. I like the fact that they appear to be a "aesthetic" solution too.

Thanks everyone!