Vx / opel corsa.

Hi all, I need a little help. I was driving the other day and my engine overheated, the cooling fan wasnt pulling in so i changed out the fan switch. But problem still there. I have a sneaking suspision its the head gasket but theres no white smoke in the exhaust and the expansion tank is boiling like a kettle. Can anyone shed some light on this?!
Thanks
Tommi