Zovirax is pretty good, but you have to put it on as soon as you feel the tingling that heralds the sore.
I certainly wouldn't be shocked at vaseline...it's great for anything wrong with your skin.
If you have any vitamin E capsules break one open and apply the oil to the cold sore. Always worked for me, but you need several applications before it goes completely.
I suffer terribly with cold sores. If you don't catch it at the first tingle and plaster zorirax on it, this stuff is the best I've found for getting rid of them as quickly as possible (stings like hell though)
http://www.boots.com/...-Lotion-10ml_1129570/