Windows 7 PC Disables LAN?

I have a bit of a weird problem; here's the whole story. For the sake of an easier explanation, let us call the PC's I have mentioned PC-A, PC-B and PC-C. So, I first had PC-A and PC-B installed (this was years ago), and the problem first started appearing right away; whenever I tried to transfer a file between the two PC's, that's size exceeded roughly 1 MB, PC-B shut down all connections (which means it lost LAN connectivity, and could therefore not connect to the internet (as the broadband is connected via LAN to the switch)). At first, I didn't think much of this, as the problem is easily avoidable by transferring files using a USB stick instead (Note that smaller files can still be sent without any problem, and the PC's even perform well in Games, when playing LAN). However, I decided to get a third PC (PC-C) up and running recently, and have correctly (as far as I can tell) installed the PC upstairs on a different switch, connected to the LAN and internet as well. However, now the problem has became a bit of a greater nuisance, as whenever I try to send files from PC-A to PC-C, PC-B locks itself down again (my guess is that this happens because the traffic is still being applied to the PC-A-to-PC-B switch, which then leads me to conclude that the problem is in the switch).

I have so far tried changing switches, changing the jack that PC-B is connected to in the switch, changing various options in the Network Adapter settings of PC-B and disconnecting and reconnecting PC-B to the network(both physically, by plugging the cable out and back in, and via the software, by enabling the LAN connection that PC-B is usually connected to as it disables itself after a while (it gets stuck on Enabling... when I try and do this)), and so far the only way to get PC-B's networking working again, is by rebooting the PC itself.Please also note, that I am able to ping between all PC's before the file transfer, that the other two PC's do NOT need to reboot in order for their network to still function properly, that only files of a filesize of over ~1 MB cause this malfunction, that sometimes (I have yet to see a pattern of this occurring) PC-B CAN ping the other PC's but has a 50% packet loss average, and takes about 3k ms to get a single response, while the regular ping speed is <1 ms, and that transferring files over wireless networks (from PC-A to a laptop connected to the switch or broadband) does not seem to trigger this problem