The problem with MOST aluminum windows is that they don't sit inside your house as far as new vinyl windows do. In order to get them out, you need to take off any siding and trim around the outsides of the windows, and then pull any nails that are in the window(leave one in the top for last so it doesn't fall out while you're pulling the lower nails!) if the window is heavier or larger than you can handle, have someone come out to help you as you pull the last nail. If you are changing all your windows, and already have your new ones, pull a smaller one in one of the less vital places in your house! Garage windows work best if they are trimmed out the same as the other interiors, that way you can pull one and see what it's going to take to install the new ones, without being pressed for time to replace them all if it's a complicated change. Change one at a time, pull one, put the new one in, weatherproof it, move on. If you have the same problem I have every time I've changed from old aluminum windows to vinyl, it's like I said, the vinyl window sits much farther inside the house than then aluminum does, and there are 2 choices you can make, you can cut your sill and the Sheetrock, to get the window to sit far enough for the fins to touch the wall so they can get nailed in. You'll do this by measuring the thickness of the window from the inside of the fin to the inside of the window(I looked for a picture but I can't find any! So how you'll do that is you'll face the window to where you are looking at the SIDE, no glass in your sight at all, and butt your tape measure to the fin(the thing you nail to the wall) and then measure the INSIDE width of the window), and then you'll compare your two measurements. If they match then, viola!, you just have to set it, putting caulking(preferably ALEX plus) on the inside of the fin, so it will make the window watertight to the wall, then checking your reveal(the way the window sits in the hole) by, on a side opening window, getting it centered on the bottom, then putting one nail from the outside in the bottom, and then opening the window just enough to where you can look at your gap of opening, making sure the gap doesn't go from a quarter of an inch to half an inch, or anything noticeable like that, and then when you're satisfied with how the window looks, sits, opens and closes, then you can nail it the rest of the way in. When we are framing, we normally have the smart guy stay on the inside to check the reveal, and then the burro(donkey in Spanish, aka laborer) on the outside, lifting the window in and nailing it after the guy on the inside approves him nailing it. On a normal up opening window, you center the window and on a new, level house, you level the top to the bottom. On old houses this typically isn't the case because the window opening won't be level from the house settling, so levels and stinglines aren't to be used! It's better for the window to match the house, a level window in an off level house does NOT look good, the WINDOWS will look out of level, because the house will be the comparison point. So when setting new windows in an old house, make sure everything LOOKS straight compared to the house, and make sure the window doesn't open unevenly(one side of the opening having a bigger gap than the other). Ok now we get to the part where things don't work all perfectly... because we know things can never be easy. When your vinyl window has a bigger gap on the inside than your old window, you'll have to do 1 of 2 things. The first, and easiest is to fir out the trim that goes around the window. So if you have a 1" difference in the gap, and your windows are wrapped in _x6 trim, then you'll have to get a piece of 5/4x6(1"thick trim!) and put it against the wall where the old trim was before you set the window, and then wrap the window again. So this will make the distance from there you nail the window to the wall, and where the sheetrock starts a little bit thicker so your window will fit. If you don't do this, then you can cut your sheetrock and sill back for your window to fit to it's depth.
Ok so that was a lot to take in at one time, but these are the 2 most common fixes that I've had to do when replacing windows, but honestly, no 2 window replacement jobs are going to be the same. You'll just have to pop one out, compare it to the new one, and figure it out from there. We never buy any materials before we go to a window job and see what the changes are going to require.