There is no way to stop lag 100% on your computer but there are ways to prevent it from happening a lot. Hardware in your computer works to run your computer making it over heat so if you have either A) lots of fans or B) Liquid Cooling System it would cool your computer significantly and lag would happen less. Lag could also be because your hardware isn't powerful enough to run the game you are trying to play so upgrading you hardware could be an option too,
use a better computer. cant have viruses
To get rid of lag go to clean up on your computer
That depends on your computer.
Your problem is in your question. With the router hooked up to the computer its using the Internet from the router which is probably what your son is using as his Internet source causing him to lag. Just make a schedule. Time him then have your time on your computer then when your done he can get back on his Xbox.
Your computer is old.
Get gamebooster
you could use disk defragmentor its in accesories system tools and/or you could use ccleaner but you need to download that
the lag is controled by the type of computer and the amount of space that your computer has! Also in your games you can add anti-lag scripts that will help do that, to add that either make one or get one from free-models!
There is none. Lag is produced from your own computer, not recording systems.
depends on how much money and time u put into ur computer, and ur connection if ur talking about multiplayer
turn it off.
either its lag on your computer OR they are at a different area than where you are at but think its probably lag