@CFCBazarcom I've changed the channel to 2. Hope it'll work fine now. But strange, other devices (mobile phones, for example) work fine, and my laptop works fine, too, when I use Linux-based OSs. It looks like the problem is not in the router or hardware at all.
@CFCBazarcom thanks for the reply. SlimDrivers have found an available update for the Broadcom driver. I've installed it, now I'll restart my laptop to check it. If it doesn't work, I change the router channel.
@CFCBazarcom Thank you for your help. After updating the driver with SlimDrivers it works better now. It disconnected only once for the whole hour. I have looked into my router's settings and found out that it already supports only 2.4 GHz, so this cannot be a problem. Do you know another causes of this problem?
@CFCBazarcom I connected Ethernet cable to my laptop some time ago, it worked normally. I use torrents sometimes, but I see this behavior even when the torrent-client is not opened (even in the background). Thanks for the Slimdrivers, I didn't think to use them, maybe they'll help.
@John I've done what you said, but, unfortunately, it disconnected again. There is no a newer version, the same only, for Windows <=8.1 (Broadcom 802.11.n)