A picture would sure be helpfull.
What type siding do you have?

 

Just added a couple pics. And I have aluminum siding.

 

There's a nailing fin on the outside of the window holding it to the wall.
To bad you have aluminum siding, It would simple to do if you had vinyl siding. Aluminum siding is near impossible to remove without damage to the siding.

 

There's a nailing fin on the outside of the window holding it to the wall.
To bad you have aluminum siding, It would simple to do if you had vinyl siding. Aluminum siding is near impossible to remove without damage to the siding.

Is that visible or do I need to pull off part of the siding to get to it?

 

Take another picture from outside.
Most often the windows go in and the trim and siding is installed after.
The only way to take the window out is to remove the siding. To do that your going to have to start at the top and remove it all to below the window.
If it was mine I would get rid of all of it and replace with vinyl siding with the same exposure.

 

Ok. I'm at the firehouse today so I'll take some photos from the outside tomorrow. Thank you again for the help.

 

I disagree with Joe on this one. Vinyl is a bigger pain because you do have to remove all of the siding. With aluminum, just cut it back to expose the fin, remove the window, install the new one, flash properly, then add some trim boards over the area where the siding was cut back. Have a plan ahead of time for what size trim boards that you'll add so you can cut the siding to the right size.
It would be VERY difficult to R&R the aluminum siding without damaging it, and replacing a section of aluminum with vinyl would look incredibly tacky.

 

That window is what we typically refer to as a "Steel Pan" window.

You can do what is commonly referred to as a frame jump in which case you would just be removing the operable portions of the window and setting the new replacement window inside the original steel jambs and interior stool/sill.

or...

You can do a full tear out by removing all of the steel frame, jambs, stool/sill, casing, etc and fully re-trim out the interior in wood. Most clients opt for this method but it is certainly less of a DIY project. Not impossible but usually outside the skillset of most home improvers.

You can cut back the aluminum siding (difficult and sometimes messy as HomeSealed mentioned) and set a full new construction with integrated nailing flange in its place. You can put a J-channel around the cut back siding and finish it out with some sort of trim detail (flat 5/4 by 4 board) or similar.

 

I do like Homeseal mentioned. I use one of those twin bladed saws where the blades are counter-rotating. Good eye protection is a must. It does leave a clean cut if you're careful with it. That's the hardest part of the job. After that, window removal isn't that difficult, and if you measure ahead and make your opening the right size to allow for your trim, it goes back together nice. Something like Azek pvc trim works nice.
Mike Hawkins

 

I'm dealing with a similar situation and thought the most common approach in this case was to remove the glass/panes and then collapse the frames by prying the frame from all sides (then cutting with sawzall). This pulls the the nailing flange out and will rip out/expose the nails. Then you can just put in a replacement window and use some kind of channel molding between the window and aluminum siding. Would this be OK, or am I way off?

 

That's about right, you just need to make sure that the replacement is properly flashed depending on what siding material is on the home. It might be a trick though given the aluminum siding that the OP has. We really only collapse the frames in brick. Other than that, the frame jump method that WoW pointed out is acceptable as well, and probably the easiest. You'll probably need someone to come out and wrap that after the install though, otherwise you'll see the old metal frame sitting there.

 

If you cut the sidng back as suggested how are you going to tape the nailing fin or install the J molding with the siding in place?

 

Its aluminum siding. You bend new J and slide it behind the siding, then caulk.

On taping the fin, you can cut the siding back far enough to allow for tape, or caulk the back side of the fin upon installation. That along with a drip cap and closed cell foam is pretty safe, especially again because its aluminum and everything will be caulked on the exterior. You have to be more careful with vinyl and the water that gets behind it... That method is at least as safe if not more so than most insert installs.