Please dont go at the car with the atitude that throwing a brick through a window and buying a new one is a good idea
The rubbers are the key points to save, unless you have new ones so make sure if you take them out do it carefully and slowly, you will regrete breaking anything.
I have removed screens and pressure on the inside of the screen in one corner and someone else on the outside with a thin screwdriver pushed in at the side under the rubber (used as a leaver) should slowly start to release the screen, its hard to explain but it does remove the rubber. The key issue it to be gental and dont go at like a gorilla.
replacing the screen is the bit you on about with the rope trick, find your self an old boy who does windscreens.
here is a good guide (apart form the cutting off the old rubber ignor that part, its the rope trick that your after)
http://www.mgcars.org.uk/cgi-bin/gen5?runprog=mgoc&a=&p=emg/mgbgtwind.html