I have a similar problem where the windows explorer process takes up a huge amount of memory when i leave the computer running overnight. It is a nuisance but is easily sorted out in the space of 30 seconds.

All i do is to kill the explorer.exe process and then start a new one by clicking 'File -> New Task' and typing explorer.exe and i am in business again.

Strange thing is that i do not get the same problem on another computer running the same version of O/S, Antivirus and a few other programs.