A runaway process can consume enormous ram, and then start to grab hd space as virtual memory.
Call up Task Manager (ctrl-alt-delete). Look at the list of processes and applications. See if one occupies too much memory. See if any programs appear as though they do not belong. One could be a virus or malware.
Some processes have odd names. If I do not recognize a name, I sometimes look it up via internet search.
There is a button saying 'End process'. This stops the process. It should also free the memory. Be careful because some processes are needed so the computer can run. If you make a mistake then you may need to restart.
A similar utility is Sysinternals Process Explorer. It displays a larger list of information.