Hi...Ihave also just bought the Edirol UA-25. I am running it on a laptop with Vista home edition basic.
Did you download the vista driver? You can get it on the Roland website. It's only a 650 kbyt file. Also, check the buffer/latency settings (in the control panel you will see the edirol icon, open it and you will see where to set the latency)). you may have it set too low.....start at 20 msec and gradually lower it and see where the pops start.