A couple of days ago, Windows Media Player started running with 100% CPU usage, practically bringing the PC to a standstill. This was constantly at 100%, even when not playing anything.
The fix? Very simply, the problem lied with very large video libraries. My videos folder is 106 GB and my torrents folder is 48 GB. Both of these folders were monitored by Windows Media Player, as well as my music folder of 24 GB. So I deleted all the videos from the library and stopped the monitoring of those folders. Straight away, CPU usage went down to a normal level and the PC was functional again.
I don't know what the cut off is for a library size. However, it was fine until I started three new torrents, each of 8 GB. So I would say that a 100 GB library is fine, but going over that, you may find problems starting the larger you go.