No, a GUID is 128 bit so won't fit in your long since that is 64 bit. A System.Decimal is 128 bit so will get you a long way, but of course that also has it's limits. Even if you save your counter in a string it has its limits. Computers alsways have limits, you will have to find one which is big enough so you can continue for a while.
edit:
In .Net 4.0 there is a BigInteger which can be pretty big, but remember, even that has its limits since it has to fit in memory.
