All of these bloggers have recycled old windows & doors to made greenhouses and cold frames. They look so great too, lot's of different ideas here:
Sparta Savings: recycled window greenhouse - made with almost no money!
The Pineapple Room: my other home - lovely greenhouse made with old windows
Maison Decor: a petite garden conservatory made out of old windows - tutorial
Kat and John: cold frame project - they make it look easy!
Coon Hollow Farm: a greenhouse is born! - and it looks fantastic
Wet Coast Gardens: recycled greenhouse - practical and creative