Okay, this is an easy enough program and I think I understand what is going on.
When I downloaded Truepiano to my normal internet computer, I had to use the soundcard drivers (i.e. DirectSound, Direct X, MME). When I used ASIO drivers - nothing. Same problem as woldsweather.
When I put this program on my studio machine with an outboard DSP (namely, Mackie 400F firewire interface) - it worked right away. I was able to use ASIO4ALL and the Mackie ASIO drivers.
Why Truepiano does this I have no idea. I have never had an issue with interoperability between different sound drivers - especially ASIO. The only thing I have ever encountered was different latency issues and ASIO is my driver of choice. Like woldsweather, my start engine was depressed as soon as I opened the program. If you are having latency issues, try configuring the bottom of the Audio/MIDI preferences. Use 44,100khz (unless you plan on recording. Then I would suggest maybe starting to purchase more studio equipment) and lowering the latency. Only lower the latency enough so you don't see any lag between your playing and sound. If you lower your latency too far, then you will get pops and cracks. By the way, that latency box is the same as buffer size.
I know this is probably not the answer you wanted to hear but, this is the first time I have encountered it. It's not you or your computer. ASIO4ALL is an excellent driver. I think there is a programming overlook in Truepiano that doesn't work with ASIO as well as it should. Again, I know your frustration and sound drivers have come a long way. Eventually, like MIDI, maybe they will standardize how they make software instruments or at least a protocall for how they interface with our comps.