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I am running XP as a guest. I am trying to get bigger window on my desktop of the xp machine but it does not work. I have enable the auto-resize guest display but still does not work. I have installed the Virtual Box Guest additions and same thing. Anybody know what I am going wrong?

Thanks

What happens if you press HOST -F while using the guest?

It goes to full screen mode but with a small box on the center which is the xp guest. I have a acer w193 monitor. Could that affect in some how?

No it just means that the guest additions have not been loaded properly. This happens sometimes in Windows.
Remove the guest additions and then reboot the guest. Then reinstall the guest additions and reboot. See if this helps.