Check if You have any adaware from Envato installed in background.
After I had bought some AEX compositions I opened an email from Envato and it installed adaware (or something like that) and some unknown tasks which were causing the problems with DL.
Also check:
nvidia drivers (if installed) , several releases (not the last one)
antivirus and firewall options
quicktime components
and try to kill all tasks You dont know (one by one)