Calm down Simguy, it’s a piece of cake for you to resize your mother-in-law’s computer by follow my advice.
I have no idea what operation system does her computer run, if it runs with win7, win8 or vista, congratulations, you dont have to download any sotware to assist you, but if not, you may need a Assistant took like AOMEI Paritition Assistant, you are supposed to read this tutorials carefully befor you take it into action.
It’s not that hard, is it? A profounded man like you will never disappoint his family members. By the way, if you dont know how to use the built-in Disk Mangement you can search it on the web, or you can still use the assistant tool to work it out.(say, win7,win8 and vista is also available for this software).