Probably a driver leak, have you got the latest drivers? Does your motherboard use Killer Internet drivers?
run anti virus & malewarescans
Download Process explorer and run it as admin (it comes from Microsoft so its safe)
the default view is tree structure meaning like your task manager screen, it will show what processes are under each service, but unlike task manager, it shows the ram usage of each part so you can see what is eating your ram
Private bytes = actual ram usage
Working set = Ram + page file usage
This page shows what all the colours and headings mean, link at bottom of it shows how to use it to find problems. You can right click processes and run an av scan from within the program.