Archives by category

I’m trying to set up archives that are defined by category. I have two main categories on my site Portfolio and Blog. I’ve tried kweblle’s plugin, which does what I need but I don’t like the urls which are: mysite.com/year/month/?cat=1

http://kwebble.com/blog/2007_08_15/archives_for_a_category

Ideally I would like my archive urls like this: mysite.com/cat/year/month/

is there any way to achieve this?

Thank you