Water leaking around window in tub/shower surround
We have an older home with an upstairs bathroom that just had a tub (no shower) with a window that was approx. 38" above the floor of the tub. We wanted a shower so we replaced the window with a new vinyl window and added tile to 64" over existing wall. Water is now leaking, down the wall into the lower level. It doesn't leak all the time, but we are now ripping out all the tile around the tub/shower surround and sheetrock down to the studs to see what extent the damage is and will replace any rotted wood etc. before building it back up. We want to do it properly. How should we go about this, and how do we seal around the window etc. to ensure this won't happen again?
I have read lots of blogs about the window, but do not want to remove it. Are there vinyl surrounds with window cut outs or should we just re-tile it? Any suggestions and/or guidance would be much appreciated.