Increasing Memory Allocation

I want to increase the amount of allocated memory in XP Mode.

I am running Windows 7 Ultimate 32bit on an i7 4930k CPU 32GRam machine.

The follow the steps below:

i) Make sure virtual machine is shut down

ii) Click Start>All programs>Windows Virtual PC>Windows Virtual PC

iii)Right click Windows XP Mode

iv) Click settings

v)Click memory 

vi) The right hand side sub-windows displays 521MB  in Memory [RAM] box and below it it says Available memory 2,477 MB

v) I try changing the value by clicking inside the box but nothing happens.

Please help!