Replacing single hung window questions..

Hi, we have a unfinished room above the garage, we are going to put some sheetrock up 12 foot sheets by 48 inches. It can only come through the window, the windows are cheaper (Ive been told) simington single hung vinyl. The company tech support says that the model we have has a stay-bar that can not be removed unless the window is removed, the screws are on the outside of the window frame they say. The house has Hardie board siding and under that is tar paper. The window has a bad bottom window sash, fogged up seal broken, its out of warranty now. Ive been told I would have to cut it out to remove it, which would make re-installation impossible. What is the easiest way to replace this window? Removing the hardie board would seems like alot of work...is there any vinyl windows that can be fastened from the inside of the frame and covered up with some fancy pop-in covers to hide screw/nail holes? Im beginning to believe there is no easy solution besides cutting all the sheetrock outside as we hang it. Maybe some trick to remove the non-removable stay-bar? and some really nice way to put it back in?

Thanks for any input.

Is the basis for replacement the bad seal or some other factor?

I mean if taking the old window out ruins it, then buying a new double hung is fine. It is half broken anyways. But if I can remove the window and somehow put it back in with the same weatherproofing and structural soundness it has now then thats ok too. I live on the coast in NC so hurricanes and water being blown in strange places is a yearly occurance. I really would like the sheetrock Im ordering to come thru the window as they deliver and will put it up there for free.

Do you have a trim board between the exterior of the window frame and the siding (most Hardie jobs do)?... If so, remove those trim boards and take the window out, nailing fin and all. If you want to then reinstall it you can, otherwise replace it (I would). If otoh, the siding abuts the window frame, you'd have to either remove a bunch of siding, or cut it back and add a trim board upon reinstallation of the window. That is we we do most commonly, however that may be undesirable for just one window as it will not match the others.
I don't know the exact circumstances of that window, but it is unlikely that the upper sash/glass/divider can be removed on a single hung to accomplish what you are looking for.

I do not think there is any trim board, all the windows on the house the hardie board run up and behind the exterior lip of the window, Im thinking nail fins and some of that weatherproofing tape is used. I took some pictures, I know this is the worst thing to do but Im thinking some screws or something can be put through the inside frame channels maybe, I would rather not go this way....

https://docs.google.com/open?id=0B7L8kxWec7jZV2FGTmd2b25QYUE
https://docs.google.com/open?id=0B7L8kxWec7jZNmtzaWFUNUhvQzA
https://docs.google.com/open?id=0B7L8kxWec7jZbFFlUVhWNzBmNk0
https://docs.google.com/open?id=0B7L8kxWec7jZcW1Ca3hFXzc5LTA

The shutters would hide some modifications to the hardie board but Im not sure about the weather proofing after cutting out some of the siding...would be nearly impossible to put small pieces of hardie back on.

fairly easy install.
remove the exising unit by cutting off the flange. might best be done with a 4" or 8" grinder with a diamond blade.
load rock.
install retro fitted aluminum drip cap.
install new replacement window.
cap and seal exterior with Quad or =.

How would the new replacement window be secured? I was hoping they made some that could be secured from inside the frame possibly. Its my main concern after weatherproofing, we get really high winds coming off the sound/ocean during hurricanes.

They generally have pre-drilled holes in the side jambs for screws. That method described is generally not preferred IMHO, but can be acceptable if done carefully. A full-fin install will have 4-5 layers of protection against air/water infiltration, while that method will have 1 or 2, so it is imperative that they are done properly.

There are typically 1 screw per corner to secure the window. You can add more screws by drilling a two step hole. Also window and door foam usually acts like an adhesive, besides airsealing and waterproofing.
Or you can do Homesealed choice:
Extract window
Load rock
cut back siding for fin install( do with small grinder), you can also do this step with the first step and save some dust infiltration to the room.
install new window with fin, back caulk the fin.
cut back the siding for flat casing trim, like Azek 5/4 x 4, install metal drip cap, install flashing tape over fin, back caulk and install composite surround, caulk edges.