WinXP using tons of RAM- where is it going?

[spoma]

I am running Windows XP Professional and lately I have noticed that when I check task manager performance that over 600MB of RAM is being used when I am not doing anything. If I reboot it drops down to around 131MB of RAM which is dealable. I have checked processes under Task Manager and even adding up the memory column it doesn't come anywhere near that number.
There must be some software that is looking up RAM and never releasing. How can I research this more?

 

[dsparks]

If you are referring to "Mem Usage: 140000K / 600000K" at the bottom of the Windows Task Manager: this figure includes swap file usage which changes depending on apps usage unless you set it up differently.