Windows – More than One Internet Connections on Windows

AFAIK "Disabled" state means that the network card is turned off. From the fact that you can't turn it on can indicate that:

• there is a problem with the driver of the card
  • try restarting
  • try re-installing newest driver from the manufacturer's site
• a different (but related) driver collides or is not installed correctly (in case you e.g. installed a firewall in the meantime)
• there actually is a hardware problem with your network card

However, I'm not sure about the "Firewalled" part.

For how Windows determines internet connectivity, see this SuperUser question. Basically, Windows tries to access a Microsoft hosted text document and judges your connectivity depending on the response (timeout, 403 denied, 200 OK, etc).

Yes, BitTorrent (or anything else) using excessively large chunks of your bandwidth can saturate your connection. Basically, you're transferring so much at once that other packets end up timing out and getting dropped. Imagine a water pipe, if you try to put too much in it will fill up faster than it can be emptied and start overflowing, losing the water that overflows.

A typical IP (Internet Protocol) request requires you to send some packets to the server you are requesting from, which will then send some packets back - its response. This requires both some upstream and downstream bandwidth. When your upstream bandwidth becomes saturated, you can't send the request. When your downstream bandwidth becomes saturated, you can't receive the response. It is entirely possible for a BitTorrent client to use all your available bandwidth.

Generally, it is best to set your BitTorrent client to only utilise up to 80% of your upstream and downstream bandwidth, as determined by speed tests. If you require low latency, e.g. online gaming, the percentage should be even lower.

It's also possible for BitTorrent to overload home modems and routers by opening too many connections, overflowing their NAT tables. The maximum number of connections should therefore be kept at a fairly low level. A maximum of 300 connections globally should be alright. An overflowing NAT table's symptoms will vary from router to router, but often cause them to freeze.