Window in this case refers to a time frame

--------------------------------------------------
Note added at 10 mins (2009-11-04 14:02:17 GMT)
--------------------------------------------------

The term therapeutic window refers to the range of doses of a drug that are actually effective in treating a particular disease. Doses below the therapeutic window are too weak to have any effect; doses above the window cause unacceptable side-effects. Doctors will sometimes say that "the therapeutic window is closed," meaning that there exists no dose that will give benefits without harmful side-effects--you're in a damned-if-you-do, damned-if-you-don't situation.

Sometimes the therapeutic window is closed from the start. In epilepsy, for example, patients are often put on anticonvulsant drugs. In severe cases, a dose that's high enough to stop the seizures will also make the patient stuporous. In other cases, the therapeutic window closes as the patient develops a tolerance to the drug. In Parkinson's disease, patients are given a dopamine agonist to treat their tremors, rigidity, and bradykinesia. For a while, it works, and they can move just like healthy people. After some time, though, this dose causes dyskinesia and patients can't control their movements. Eventually, the drugs will either do nothing or cause involuntary movements, depending on the dose you give.

http://everything2.com/title/therapeutic window