Rootkit hidden boot sector removal!!!!? :/?

I have avast and it told me that i have a rootkit hidden boot sector and its reallly screwing upp my computer i hate it and i have tried every single thing out there to remove it... even malware bites butt it tell sme that i need money to pay for it to remove it ... is there anything out there that is completely free to remove it where you have to pay nothing!!!!