Hi
It looks like the problem is solved. Yesterday there was a windows update available: "Atheros L1 Gigabit Ethernet 10/100/1000Base-T Controller" so an update for my network driver. I've installed it and the problem hasn't occured since (downloading about 24 hours now)
On another windows 7 forum I read there was somebody with almost the same problem. He had a windows freeze only when browsing, so not with other internet apps. His problem was also solved by a windows update for his network chip.
Checkdisk also repaired my data drive. But I don''t think this is what caused the freezes because I was able to copy large files to that disk without a problem.
I'll keep downloading for a while to see if the problem is really solved, if...