The windows XP tasks manager can show two different columns regarding the memory usage of the processes. One is called Mem Usage and the other is VM Size (not on by default, you need to activate it)

From what I've gathered, VM size is the size of the entire memory space occupied by the process and Mem Usage is the amount of memory currently committed and used. This assumption is verified by most processes when the VM Size is only slightly larger than Mem Usage for instance my Outlook currently has 79,724 K in VM Size and 56,600 K in Mem Usage
But it fails for other processes such as Firefox which currently has 171,900 K for Mem Usage and only 156,440 K in VM Size. How can a process use more memory than the amount of virtual memory allocated to it?

So Maybe my interpretation of these columns is wrong. What do they actually mean?