Just a quick warning for those who wants to play with PC oscilloscopes.... of course It's nowhere as good as TEKTRONIX oscilloscopes
but also very easy to damage once you supply the wrong thing
I once found a DOS program which provides a parallel port oscilloscope via a 5V ADC. I mistakenly supplied 15V to it - surprisingly the ADC was not damaged but the parralel port was! Luckily I did not use the built-in port, I use an ISA card for parallel port so replacing it was easy. When the card was taken out, I noticed several chips were severely burned...
At another time I used a sound-card oscilloscope (input via microphone). I was designing FM transmitter so without thinking about the supported recording frequency of a sound card (max. 44.1kHz), I attempted to see the waveform from the output of the RF oscillator. The computer immediately restarted, and after that I found my sound card to be "deaf" - playback was still ok but recording could not work. Probably the too high frequency from the oscillator had killed the microphone part of the sound card?
I always use proper oscilloscope for experiments. To me, PC oscilloscope only works as proof-of-concept.