If you get a virus is there any way to find out where it came from so that site can be avoided?
Yes but this is likely way beyond what you are capable of (no insult meant, it's something I would have a very hard time with myself depending on the circumstances and that very smart people are paid a lot of money to figure out).
A better option is to find out how they are getting onto your computer, because they shouldn't be able to do that no matter what site is doing it.
I get the same trojan now and again my AVG bags it but next week I may get the thing back again
It sounds like it might just not ever be removed completely from your computer, or your computer still has the security hole that let it in in the first place. You may want to have someone look at it and make sure that the malware is removed and the computer secured.