In reply to layback larry:
I was in Peru from beginning of November until Mid-December. The rainy season started as we were in Cusco region towards the end of November, and by the time we reached the Cord Blanca (1st week in Dec), it seemed well established, with regular rain in the afternoons, which limited the views whilst we were trekking. However, this could have also been an unusually wet couple of weeks, as I believe it is possible to trek at this time, with better weather. I suspect we were a little unlucky, but did expect this to a certain degree, as we were tied to going at that time of year instead of July/August which is reputed to be the dryest time.
We then headed N into Ecuador from mid Nov until early January, where the weather seemed generally more settled and fresher, and free of the coastal mist/haze in Lima. Many trekking companies actually choose to schedule their Cotopaxi/Chimborazo trips over the Xmas period as this usually guarantees reliable weather, although I gather folk will head there all year round. Sept/Oct should be OK, although possibly a little warmer than the (UK) summer months...
have a good trip!...