free s pace on computer

Hello: I have just serviced my computer and upgraded from 1 g to 4 gs.
Everything is so much faster but the computer guy is dumbfounded as to why my
free space under where is says physical memory drops to 50 and below,
something is taking up that space, from over 2000 you can see is drop to 0
right before your eyes. can you help us figure this out....Thank you