Okay, some basics here.
You can't turn down the input gain in Audition because, by the time the audio reaches Audition, your computer has already converted it to a digital signal and any clipping from high gain is already done.
Second, how exactly have you switched to ASIO? Not many built in sound cards come with suitable ASIO drivers. Assuming it's a Realtek or similar in your HP, it'll have MME drivers and you must be using some third party drivers. That could be your problem.
Third, how to set levels (sort of): Assuming you're on Windows Vista or 7, open your Windows audio control panel. Go to Recording Devices. Select the input you're using, probably Microphone or whatever. This'll open another window with tabs across the top--click the one labelled "Levels" and adjust the two sliders. However, be aware that, even at this point, you're adjusting the digital conversion so if things are overloaded at the input you're still out of luck.
Fourth: An Audio Interface is a proper device for recording use (as opposed to built in sound cards which are for Skype phone calls and playing back videos. There are FAQs about these in the newbie forum--but you'll probably want one with a USB output and a knob on the front to adjust input gain.
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