I spent the weekend installing windows at one of the homes I purchased last month. I had ordered 24 windows and they are made custom for you - since I measured them, I owned them. I put the first one in place and it got stuck - could not get it in or out. I was sure I had just wasted a lot of money. Fortunately, with the help of a block of wood and I hammer I got the window in. Afterwards, I figured out that I could trim an eighth inch off the bottom and the windows fit OK.
A lesson I learned is that 1) windows frames are not square. One side was consistently shorter than the other 2) you need to remove the windows in order to measure them properly. I tried to measure the vertical dimension with window in place and I think the tape measure added extra length by bending around the window panel, so all my windows are too tall and 3) with a little bit of resourcefulness, you can solve most problems.
I installed 13 of 24 windows this weekend. I can get a window installed in about 40 minutes. It's not really too hard of a job once you figure it out.