Depends, where are you downloading it from? I've got several program which will detect a virus straight away/even a malicious link and prevent me from downloading it.
I personally like:
Ad-aware remover (the new one)
Usually i can't open the software if its dodgy in any shape or form.
If i'm pretty sure my computers infected then a program called Trojan Remover by Simply sups (I think) is amazing.
"Train hard or don't bother training at all!"