Hello mryumyum and welcome to Seven Forums.
As you've indicated, if the PC has been in extended continuous use memory leaks can occur from some programs (usually badly written ones.) That means used memory is not released for reuse. Over time this can accumulate. If, after each reboot, your memory usage drops to the same level, I wouldn't worry. Rebooting more frequently might help if it's the result of a badly written program.
As a general rule, you should not have any available RAM (or at least very little). Available RAM is wasted RAM. You paid for all of it and shouldn't want to see any of it wasted. Windows 7 works hard to keep all your RAM working for you all the time, for example using it for cache if your apps don't need it, then taking it back again if your apps need it later. This is a good feature of Windows 7, not bad. You could run the Resource Monitor to see exactly how your RAM is being allocated. Resource Monitor
Another possibility would be malware. Make sure your security software (including real-time protection) is present and up to date. Do a full scan of all drives for malware with your installed antimalware suite. You could then follow it up with free Malwarebytes
for a second on-demand scan since no antimalware product is 100% effective 100% of the time.
You could also run the free Process Monitor to get a better idea of what's using your RAM. Don't even have to install it. You can run it from the live link at the bottom of the page (run process monitor now.) Process Monitor