Hi, I have to start by saying I'm no computer expert, I was hoping some geek might chip in, but that said it seems Skype is the problem, system idle process is actually the amount processor power not being used at that time so showing 85% is quite good, don't think adding more ram would help with this problem, perhaps reinstalling Skype might be you next move, also typing "skype high cpu usage" into google comes up with quite a few hits.
Good luck
