Tom before you start thinking about the head gasket and with the icy weather we've been having have a look at all the cooling pipes very carefully, particularly the joints. Look for any signs of fluid leakage and check the tightness of the joints. The next thing to check is whether or not the water pump impellor is still attached (There are threads on here that include a check on how to find out). Although a blown head gasket would give you similar symptoms, a leaky pipe will do the same due to the system not pressurizing correctly and therefore the water starts to boil and produce steam. Thats one reason you shouldn't remove the water expansion tank cap on a hot engine.