Files are on our webserver and haven't been copied, except of course when we downloaded DNN zip and extracted files here.
On file properties, under General tab, file isn't checked as hidden or read only. On advanced tab, there is check for archiving and indexing which isn't related to security and this problem.

I think that there is something with security tab on file, but group everyone is allowed full control and Network service also...

Thanks,
Ivan

We use Server 2008, so Network Service (old name ASP.NET) has Full control over that folder and the rest of DNN folder...