Mar 3, 2011 - Windows 7 reports how much physical memory is currently installed on your computer along with how much memory is available to the operating system. Of the total physical memory minus “hardware reserved” memory. Advetisements There is a clear difference between the then term installed memory and usable memory. Windows 10 verifying your download. Amnesia dark descent mods. The term Installed Memory refers to the amount of physical memory installed on your PC, whereas usable memory refers to the amount of memory that can be used by windows ( or what is available for windows after sharing some of the installed memory with other hardware ). Jan 28, 2006  Reload this Yelp page and try your search again. Total physical memory versus available physical memory? I understand that. I guess my question is, what would account for the low level of available memory? Too many programs loaded in the system tray? Anybody know? I just want to avoid having to go to my computer tech dude and spend 75/hour. Jan 22, 2008  Best Answer: Total physical memory = Amount of RAM installed on a pc e.g. 1gb Availiable physical memory = the amount of free memory which isn't all ready allocated in cache or running programs.If you havn't got enough availiable physical memory then you need to terminate some programs which are not.