It's author has gone on to develop ZelScope, which is not freeware.
WinScope is limited by the sound card; you can only get 20Hz to 20kHz response. Forget about DC levels, because the sound card's mic input is capacitively coupled. However, it does have a decent spectrum analyzer for the audio range; something that you'd pay a lot of money for in a hardware solution.
There is also a very real risk that you can damage your sound card if you're not careful.
There are better hardware-based solutions out there, with higher frequency response/better bandwidth - but you'll have to build the hardware.
Here's a page that has a relatively simple 8-bit ADC to parallel port interface - but that just gives you a single channel. http://www.geocities.com/lptscope/
There are other links from that page.