Look for any settings related to automatic gain control or audio enhancement in the control panel and disable them. These settings are often on by default which makes sense if you plug in a microphone and use it for something like Skype, but not so much for recording music.
You now have to be more careful about watching the input levels and decrease the recording volume to the point that the audio doesn't clip.

Hey, while recording the latest bits of my music I stumbled upon a problem. When I record just one high pitched instrument everything is ok, but when I add some bass, the high pitched instrument volume decreases a little bit. You can notice it HERE. I wonder what's the reason for this. Is it my soundcard because I use a crappy integrated realtek circuit or maybe it's the drivers (directsound, xaudio,asio)? Anyone had problems like this?

PS: The funny thing is that I listen to the track while I record it and it sounds ok and the problem comes up when I playback the recording.