Dabossa, Casually... ¿do you have big exe files or big compressed exe files in this folder?
I have little problems of 100% CPU usage when I try explorer my folder of downloads because this kind of files, with a manual update of Avast seems to have be resolved.
Try move this kind of files to another folder and try again explorer or navigate through your folder of download to see if the 100% CPU problems persists.
Greetings !!!!
EDIT EDIT:
¿Do you have directs links to folder or files that are shared in other computer of your network?
Specially, in the desktop, if you have this directs links and your remote computer is offline the navigation by explorer may be very slow. I had problems with this in past.