What do the columns that are not shown by default in the XP Task Manager do? Specifically the VM Size, Page Faults and Mem Delta columns
VM Size (from What does “VM Size” mean in the Windows Task Manager?)
That means the whole size of memory a process allocates. It is the size of physical RAM plus the virtual memory size. Basically, the more VM Size a process allocates, the more resources it consumes. Processes with high VM size and small RAM size will often have page faults (see below).
Further reading material: An introductory guide to Windows Memory Management
Page Fault (from Wikipedia)
So basically, whenever a program requests something that is not in the real physical memory, a page fault occurs. The page has to be loaded from the virtual memory, which means longer loading time. To be more precise: Higher page fault values indicate poor performance because the virtual memory essentially is on the hard drive. As the hard drive has higher access times than RAM, the process needs to wait longer for the requested data.
Still page faults are very common because the RAM can't keep everything.
Mem Delta (from Microsoft TechNet)