In reply to sbc_10:
I strongly suggest Veloz Coronado Expediciones, based in Riobamba.
http://www.velozexpeditions.com/
email info@velozexpeditions.com
I spent some time talking to Enrique (who set us the business but does less guiding nowadays) and then to his son Ivo (who speaks good English) and I liked them very much. They both love mountains and mountaineering, are extremely experienced, and just want others to enjoy them too. (In contrast to the impression I got of other companies.) They were honest and this well established family-run business is exactly the one I would choose.
I hope you go for it and have a brilliant experience.
Do make sure you are acclimatised first!