The performance monitor is not correctly representing the usage. It thinks you have two physical CPUs, so if you're using all of "one", that's only 50% of the total. With HyperThreading, what is more interesting is to see what level of use there is on the "other" processor by other threads (in other processes, in your case.)
Your DOS program is single-threaded - it cannot use more resources than one processor can supply. HyperThreading doesn't really give you two processors - it lets the OS run a second thread while a first is stalled (waiting for memory or I/O, for example.)
