I mean physically moving the mic to a different location further away from speakers etc lol, it's a very long shot as it doesn't really sound like feedback.
If you go into the windows volume options and click advanced at the microphone, can you turn down the treble manually? it may not fix it entirely, or do anything, but there's a chance it could help.
I presume your sound card drivers are up to date, do you have a separate control panel for the card?
edit: do you have a second microphone socket? Mine acts funny like that, the rear socket is quieter than the front, even though they're both set up the exact same way
This is my sandvich. There are many others like it, but this one is mine. My sandvich is my best friend. It is my life. I must master it as I must master my life. Without me, my sandvich is useless. Without my sandvich, I am useless. I must eat my sandvich true. I must shoot straighter than my enemy, who is trying to kill me. I must shoot him before he shoots me. I will. Before lunch I swear this creed: my sandvich and myself are defenders of my country, we are the masters of our enemy, we are the saviors of my life. So be it, until there is no enemy, but sandvich. Om nom.