If your PC have not installed any instrumentation card, you can use just the soundcard.
A very powerful and commonly used program is Virtins Sound Card MultiInstrument (you can download it at p2p networks). It can be used as function generator, oscilloscope, spectrum analyzer and more (in audio band, of course).
If you need to work at higher frequencies, a cheap solution is to purchase a PC-bases USB oscilloscope (I have this one **broken link removed**,
and it works fine). It has a spectrum analyzer and can be programmed with an API.
I hope this help you. Regards.