If there is a trojan nestled somewhere on your pc with an odd filename...can a firewall still prevent someone from using it to get into your computer?

Also, I ran a trojan and full virus scan both of them scanned and said there were no trojans or viruses on my hard-drive. However, yesterday Norton detected one and was unable to delete or quarntine it. So even though I have done two major scans with two different pieces of software and they said there were no trojans/viruses, I'm assuming the virus that orginally was detected is still there. Should I still format my hard-drive and heed the first warning I got or should I wait a bit or just forget about it since I ran two system scans after this and nothing was detected? Could it have been an error?