Hi Benonidoni,
When you are in the Virtual PC/Windows XP mode, go to Tools and click Settings. If you do not see Tools/Settings at the top bar of the Virtual PC/XP mode, minimize the Virtual PC screen.
After clicking Tools/Settings, click Memory in the left half of the form that pops up. I think that here you should be able to change the allocation of RAM.
Ciao,
Hviezdoslav