I have that problem too. It is due to aging. The family promises me I can
hide my own Easter eggs next year.
Seriously, what are you looking at that tells you that you have 500mb less
memory than you used to? Memory is the chips inside the computer where your
programs do their work. I doubt you mean that.
If you mean space on your hard drive, that is not memory, that is storage.
If that is what you mean, an accumulation of 500mb of temporary and other
files daily is not unusual. Have you run Disk Cleanup lately? Type 'disk
cleanup' in Start/Search and hit Enter. (Disk Cleanup is located under
Start/All Programs/Accessories/System Tools.)