Something lives in my PC (HW #7)

Although Symantec's soft ware and free Spybot show that my PC has no virus, Ad-aware find out MLU lists and asks me if I remove it...each time I run it. What is going on? Has something been working on C drive? On the internet, I searched every word ad-aware told, but I could not find that the cause must be virus. I worry if the stranger inside my PC would a new type of virus or spyware. Even if it will so and I format C drive, the big problem is that I do not know where I got it from. I will visit the site and may get the stranger again. Hum...what should I do? Moreover, I am surprised at Symantec's soft ware and free Spybot that could never find the new stranger.