My computer has a trojan. How do i get rid of it?

hey people. my computer has a trojan (Troj/TdlMbr-A) on physicaldrive0. my webroot detects it and says its quarantined but when i go to my quarantine it's not there. When i scan again it reappears. i've tried using malwarebytes and it detected alot of things but not the trojan. My computer screen messes up. please i need help FAST! for free too.