Many save a lot of stuff to their desktop, which uses up the allocation for RAM.
RAM is volatile so that when there is no power to the PC/laptop, it loses what was contained in it. So if the Desktop contents
were stored in RAM, you'd lose them every time the PC was rebooted or switched off. Unless you have configured your Windows installation differently, the typical place desktop items are stored is somewhere like C:\Users\<username>\Desktop. i.e. it is NOT in RAM, it is on the default hard drive.
Loss of available RAM can be attributed to several things, including memory leaks (software uses up RAM in order to run but then doesn't return it for use by other programs when it is done with it), background processes running such as Windows Updates, running Windows on a machine with not very much physical RAM, running Windows with a smaller than expected virtual RAM setting (page file).
One place to start to free up RAM would be to save any work, then shut down and restart.
