An infected machine or one that runs torrents. The hunt begins.
The other reason is that when Windows PCs are off for about a week, when you fire them up they could be downloading a few gigabytes of updates and will finish when they are done.
I moved into a house wired with lan cables going from each room into a central box. My cable input also goes into the same box. When I was using my cable input to my modem to my router and using wifi only, my internet speed was very fast. Once I plugged in 3 cables from the rooms I planned on using wired internet, along with the cable labeled 'feed' into my router, my speed slowed to a crawl.
Why did this happen, and how can I fix it?