Explore.exe memory usage keeps increasing over time?

just like the title states, when i first boot up my computer explorer.exe stars out at 15,000 K, as time goes it increases, one thing that bothers me is when i open a folder or windows explorer, memory usage increases, which makes sense, but after i close it memory usage does not decrease, it remains the same, so the more folders i open over time the more memory is used, which is really scaring me, my computer is not slow but i would like to know if this is normal, before i rebooted my explorer.exe was at lik 65,000!!! now it is at 40,000 but i am not sure how long it will stay that low, can someone offer me a solution to this problem, or this normal? i have a toshiba setellite L505D and 3GB of RAM. my computer already as a problem, EXTREMELY SLOW booting time, when i restart or even log off i get s black screen for like 5 minutes before being taken to the log in screen, can this be related to the explorer.exe issue?