all downloads stop after a few seconds

Since a few weeks all my downloads on a Dell Inspiron 620 stop after a few seconds. I have 5 PC's at home and only this one is having the problem. I tried both wifi and fixed line to the router, I removed Advanced System Care, disabled Norton Antivirus and Firewall, etc etc. I tried it from IE as well as Chrome, same problem. I also have it in programs e.g. Java related downloads, or Spybot updates. Any tips? Didn't have it in the past so must be software related. As I tried it on the Dell wifi card as well as the build-in realtek ethernet card I guess it's not a driver issue.

I'm using Advanced System Care also on my other PC's without issues, so it sounds unlikely that this is the cause

Thxs, I performed the cleanstart and discovered residual McAfee services, deleted them using a McAfee uninstaller, but the problem still persists. After a cleanstart I cannot test the download function as there is no network, is there a cleanstart function that still enables a browser and network?

The full uninstall of McAfee was succesful using the McAfee unistaller, so that's off the PC, only Norton is there now.
When I boot in safe mode I do have network, but can't open browsers due to proxy issues. Does somebody now a test download (of at least 10Mb) that I could try in safe mode? Or is there a way to make browsers work in Safe mode?

I ran the Adwcleaner, it did some cleaning but to no result, downloads still stop after about 5 seconds. Furthermore the network connection is excellent and online games etc work perfectly well, just downloads taking more then a few seconds will abort. Any further tips?