I am in Canada.
How much should windows 7 be?
I saw some are 70$ and some are 140$?
Can anyone help me to find a good website/store to buy it? ( I am in Canada)
There is different versions depending on what you want to do. As you didn't even knew this then it should be windows 7 x64 home edition. I would recommend Windows 8 x64 bit though. It has way shorter booting time and overall better performance due to the lower system requirements.