I have downloaded a trojan horse remover but I know when I start the scan my computer will freeze or reset itself...
Please don't download or run anything for now other than whats recommended here.
Your problem seems to have occurred when you ran AVG Anti-virus. Where did you download it from?
I want you to download and run a programme called Combofix.
You must follow the instructions at this site carefully for it to work properly. When you have the log please post it here and also give us a report on the current situation.