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Seal around all 4 sides but DO NOT seal the gap/slot between the window and the cill, it is there for a reason, namely drainage 
When it's raining you say! And window's plural! So not an isolated leaking window!
Sounds more like condensation forming on the cold surfaces around the windows and running down onto the internal window boards. A lot of condensation can look like a leak.
When it's raining you probably have the windows closed, its colder outside, you put the heating up, there is a greater difference in internal and external air temperatures and with the windows closed you probably don't have ventilation so get condensation.
I may be wrong, on a dry day do a soft pressure hose test to replicate rain and see if they leak.
Sounds more like condensation forming on the cold surfaces around the windows and running down onto the internal window boards. A lot of condensation can look like a leak.
When it's raining you probably have the windows closed, its colder outside, you put the heating up, there is a greater difference in internal and external air temperatures and with the windows closed you probably don't have ventilation so get condensation.
I may be wrong, on a dry day do a soft pressure hose test to replicate rain and see if they leak.
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