[wubi] Can i expand space to ubuntu

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http://ubuntuforums.org – hey guys i installed ubuntu with 20 GB space using WUBI where i installed Windows on LocalDisk(C) when i opened ubuntu it became slow very slow. If i opened a terminal and say firefox with 5 or 6 tab's and geditor and if i switched between them fastly the system is getting struck. I don't know why can anyone tell me why and how to resolve this?? and can't we assign More than 30 GB when we are installing ubuntu using WUBI?? Thank you (Hardware)