GUESS WHAT I AM

I always seem to be opened whenever you're hot
I'm also firmly closed and locked when you're not
My shape can vary from being square or round 
Houses or buildings is where I'm usually found 

I really don't like being cracked or broken too 
Now can be found on your computer is also true
Many of you gaze out of me either day or night 
Thank goodness I have a frame keeping me tight 

So no more clues given about my secret identity 
Was first around I'm told In the thirteenth century
For I am dear friends a most humble window 
Always reliable and available I'm sure you know