I couldn't take it anymore with f-secure and the wmv files, and switched to avast. everything running as it should, no delays on playing wmv files.
but, I checked and can say that avast is not scanning on-access the wmv files, only on-demand. this is why there is no delay in playing such files. if avast is happy with this, I'm happy too. just for information, the scanning (on-demand) of a 700MB wmv file takes around 30sec.
the only thing I will miss is the Deepguard in f-secure, I don't think the behavior shield in avast is a match for it.
thank you, David and scythe944 for your replies.