I was in Vienna and it poured for 5 days straight. Very disappointed. When is the best time to go?

Kaka - I have friends who went in mid winter and loved it

Steve

I was there in May 2006 and the weather was perfect.

I think you were just unlucky. I don't know that anyone can guarantee you a "best time to go". Weather is unpredictable.

I am soory, but in central Europe, it rains 150 days per year, evenly distributed across all seasons. This is the reason why Wien has so beautiful Cafés, Beisls and museums.

April/May or September/October.
December if you are into Christmas markets.

The chances of having no rain are not much better than in any other month, but it's neither too cold or damp nor too hot.

I have been to europe over 70 times - some business and some vacation, with trips lasting from 6 days to 6 weeks (mostly were in the 2 to 3 week range). With the exception of one trip primarily to southern Spain it has rained at some point (and in some cases very frequently).

Weather - and rain - is a fact of life. Unless you're planning on camping out or doing only hiking it shouldn't really change your plans much. Just be sure you have a sturdy umbrella and waterproofed shoes for every trip - and you won't need to worry about the weather.