I am using Windows Vista and I installed Bicomet 1.04 and it has been running fine for a few days, now all of a sudden its just keeps shutting down for no apparent reason. Does anybody know what it might be.

maybe it's a system failure. (overheating or something). Maybe you wanna check the fan inside the cpu.

And if you haven't try it, try uninstalling BC. If it still does not give a better result, I think it's a virus that causing all that.