Getting a Trojan AHD.2. and Security Risk Processing twice now!

I have been using Norton 360 for several years now I just loaded a new version on to one of my computers and since I did I have got something like this

Auto Protect is Processing a security risk Trojan AHD.2. and it locks up my computer. It has happened twice now since I loaded the new 360. In the past if it detected anything it would "Block" It but so far this has not happened. and when I run a Full Scan it cannot complete it something locks it up and it never finishes last night I started a scan and this morning it was locked up what is going on and why can I not do a "Full Scan"? Never had this but since I got this Trojan this computer is not Reliable if it runs for maybe 5 minutes then locks up. I let it run the "Processing the Risk and it stopped for 2 days and then it return. In the past it would stop the threat instant by blocking it or quarantine it but now it just shows it is processing it and when it is I cannot do anything everything just stops working? Never had this with a Norton Product before? Why can I not do a full scan??

Matthew02