Mum,
When Avast generates the report, it names the path to the infected file. This is important information because you can "follow" the path from the DOS prompt, or through Windows and delete it manually. Then you can run Avast again to see if there are any "leftovers".
The fact that the pc freezes and gives you the "error in deleting" message does indicate that the virus or trojan has now become a process as Newey suggested.
Stopping the process and rerunning Avast should allow you to delete it.
We can offer recommendations on Trojan and Spyware prevention later on after things are cleaned up.
Let us know how you do.
Techie