System Cache keeps going up?

[dieselstation]

my xp system is going weird on me. my System Cache keeps going up and up and my Available Physical Memory keeps going down and down. Is this normal? it changes every second. This is when i do a Cntrl-Alt-Del under WinXP and look under the Performance tab. argh.. what's leaking all my memory? the only thing i have open is IE, Serv-U, and Norton Antivirus Autoprotect thing.