Materials
Boats built 1980's and before may well have chromed brass screws/machine screws which are quite soft, but suffer less galvanic corrosion. Later fastenings are likely to be stainless steel which may show more corrosion against the aluminium frame.
Being prepared
Patience and planning are very important when tackling old screws.
Check that you have a screw driver that is a perfect fit to the (probably slotted) screw head. If not buy a new one as using one with a poor fit will likely tear the head.
Get a heat gun (and power), preferably one with a narrow nozzle.
Have a supply of releasing fluid or if not white spirit or similar and some scotchbrite pads or similar.
Have a hammer, a drift/flat punch of diameter similar to the screw head and a centre punch to hand.
Have a cordless drill with a selection of metal cutting (eg cobalt) drills 2-5mm, just in case.
Have a small supply (can) of light oil.
Have a container for the screws.
Know where your nearest fire extinguisher, or fire blanket is located, just in case and be sure you know how to release and operate if need be.
Procedure
Before the screwdriver...
Clean out the slot in the screw head and remove any obvious corrosion around the head as best you can (scotchbrite is good for this with a little white spirit).
Heat the area around the screw, making sure you don't overheat any surrounding GRP or other combustible material. The Aluminium and brass/stainless steel will expand at different rates which will help to break any corrosive bond.
Apply some releasing fluid
Take the hammer and drift and give the head a few sharp whacks to again help shock release any corrosive bond.
Apply another dose of heat.
You're now ready for the screwdriver
Double check you have the correct screwdriver the tip of which is the same width as the head and that the blade thickness both allows fill penetration in the head but with no looseness. A loose blad will apply a point load to the edges and tear. A blade of the correct width will apply load across the head.
Orientate the screwdriver parallel to the screw and apply inward pressure before the turning.
If the head turns, great, if then with less pressure even better. If it turns but is still really stiff try a few small loosen/tighten cycles and then loosen a bit more. If still stiff consider running through the earlier procedure again as (particularly if brass) there could still be considerable risk of shearing the head/shank. Again, BE PATIENT!
Oops, the bugger split/sheared or is a head mess
If the head shears you can normally still release the frame and deal with the remaining stud later.
If the head slot is mashed you'll need to drill it out using the following procedure;
Clean the head if necessary to give a good surface to hammer into.
Take your centre-punch and put a decent indentation in the centre of the mashed head. This has to be in the centre as it will guide the drill.
Take the cordless drill and add the 2mm drill bit.
Put some light oil on the screw head - this is to help the drill bit, reduce blunting and extend the drill bit life. If you don't have oil use white spirit or similar.
Place the drill bit against the indentation, check the shaft is parallel with the screw and then SLOWLY, drill to a depth just below the head depth, cleaning and replenishing the oils as you go.
Be VARY CAREFUL to ensure the drill is in the centre of the head as any deviation will see later drills slipping off the screw shaft into your lovely soft aluminium frame!
Repeat with larger drills until the head pops off. Note that sometimes increasing in small drill increments can lead to initial snatching. If so try larger increments, but don't forget the oil as you may need to repeat for other holes.
Place all the screws in the container. Note any variation in lengths and if found mark the size and position on the frame using some masking tape. Though most windows use one length, this is not always the case, so be on the lookout.
Hopefully you've now successfully released all the fasteners, even if some need remedial action later.