A friend of mine did hamachi.cc from start to the point where it was acquired by another company with 4 million users. That took 1.5 years. He did everything - the UI design, coding network interface drivers, the server, marketing, support, ecom site and business development. So, yeah, it's doable, and, yeah, the workload is ungodly, and, no, he didn't want a co-founder.
