@Kaje: You may need to add an exception for the temp folder as well as some trainers are extracting necessary files on start that may be blocked by Avast. But the easiest solution would be uninstalling Avast completely.
I haven't experienced that as yet.
I love Avast! and it's still probably the best free anti-virus and anti-malware program available, for me I'd rather add the simple exceptions than get rid of it.