XP pro memory usage

If I reboot the system and view the task manager, it
shows just over 180Mb memory used.

If I repeatedly run and exit out of a large application
(I am using AutoCAD) the total memory usage goes up and
down by about the size of the application - just under
30Mb. I see no sign of any of the 180 Mb being used for
caching.

I have a collegue with an XP Pro system that shows about
70Mb memory used after a reboot. On my machine, if I add
up all the processes displayed (with the 'all users'
option ticked)in the task manager, they come to about
70Mb, but the total commit charge is displayed as 180Mb.

What else could be using my missing 100Mb?

Cheers,
Caspar.