To me the steam software looks like a mem leak sucking more mem than needs . Thinking from excesive processor cycles and lack of Ram could be but asumming he already closed unneed junk then that maybe is not either.
Have you tried updating the Steam client???
And maybe you should also check the CPU/sensors temp. When the heats gets high (usually 70 Celsius or more)] the pc start to get slow and freeze even with the most basic things. If so clean the dust with compressed air or similar . Most important are the processor fans and the powersupply one..
http://www.almico.com/speedfan.php
That is a free soft that could help to monitor you fans.
Good Luck