Where is all my physical memory going?

I am curious as to where or actually what is using all my physical memory except about 27%.
I have 256 MB of RAM installed.Running XP Home Edition. I shut down most of the applications I HAD running in the backround. Before I did this I only had 19%. Is 30% available normal? Also, does NOT having more available Physical memory slow my machine down? System is HP XL916 Celeron 850 MHz.