[MovedGoalPosts]

There is a firewall within the router so that will offer you a degree of protection. You do however need to get antivirus loaded a.s.a.p. Especially if you are downloading from various sources. Can you be absolutely sure that the sites you are visiting are not infected in some way?

As for the large downloads, has your network got anything else running on it during these times? If there is anything trying to upload, you might be saturating your upload bandwidth which is then stalling the router.