There are a few things to try:
Try VLC player, it is less demanding than Windows Media Player. Also, make sure your hard drive has some spare space ~10%. If it is full, performance will be crippled. Defragmenting the hard drive may help a little bit to.
Open task manager and check individual processes. There still may be a background process running. If the player is above 80% there is not much you can do but upgrade.
Give your pc a good clean, all heatsinks, filters, meshes. And if you are willing, redo the thermal grease. This will not reduce the load but at least the temperatures should improve.
If you are willing to upgrade, I would get an i3 and a H77 motherboard to go with it.