[Hoags]

The biggest problem you may have is getting the old winshield out of the junkyard car unless they have done it for you. If you get a used glass like Xero says make sure it is clean, especially where the old sealant was. You are going to have to get it all off do not assume that it is clean enough, even a slight bit of old sealant can come back to bite you in the butt later in the form of a leak. Just make sure to take the time to clean both the glass and the car inset you will be better off and run less chance of developing a leak later.

If the scrap yard window is still in place I would see if the car can be put in a warm area (If you are in a cool/cold climate) let the sealer get warm and remove the trim, if you do not have windshield tools you may be able to use a piece of heavy cord, find a way to work it between the glass and the car frame through the sealer and have someone help you work it back and fourth as you move it through the sealer and the carefully push out the glass as you go so the sealer does not re-adhear as you go.
Good luck.

P.S. Check your auto ins. Most have windshield replacement with a small deductable. It may be worth the phone call. That way you don't have to mess with anything, hell if you do have the coverage most places have mobil units that come to you and do the replacement.