With regards to the Microsoft Security essentials, it will continue to function but will not get updated.
Two options are:
Upgrade the PC to Win 7.
Install AVG Free from this link:
http://download.cnet.com/AVG-AntiVirus-Free-2014/3001-2239_4-10320142.html?spi=6a5298bac593dc853b6646da1b988d3f&part=dl-avg_free_gb

Choose Basic Protection and then CUSTOM install, remove all the ticks from the additional components.
It will need a reboot at the end.

Then you can remove MSE.