It’s nice to hear from somebody that wants to preserve the life of a spider (instead of asking for advice on killing them). Your garden spider is most likely a female who has laid her eggs and is now just standing guard over her egg sac until she expires. Depending on what species you have, the eggs may hatch now, or not until spring. In either case, the young spiderlings will remain in the egg case until the first warm days of spring when they will set out for their own six months of feeding and growth. These young spiders will probably do quite nicely in your sealed windows, you may even get a chance to watch them disperse when you remove the plastic in March.