detecting a trojan

[12 gage]

I had a file that I dl'ed for a p2p it was in my files for at least a couple of weeks. Then during one of my nightly scheduled norton sweep suddenly it says that the file contains a trojan. But for weeks it was checked and missed. Is there a better trojan search program and a program to remove them. My computer seems really slow and I'm wondering if it's infected?
Thanks in advance

[Kf7avenger]

U Might want 2 Try AVG its a Free antivirus that also hasMore Trojans in their List cuz i had bout 300 trojans an i had Norton But Norton never found them until I got AVG which Came in with my PCWORLD MAgazine U might also want 2 try ANtIVIR another FREE Antivirus or AVAST

AVG http://majorgeeks.com/download886.html

ANTIVIR http://majorgeeks.com/download955.html

AVAST HOME http://majorgeeks.com/download1968.html