I'm dual booting win7 and ubuntu12.10 and ubuntu thinks there's no space on the hard drive

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http://askubuntu.com – I'm dual booting win7 home premium and ubuntu 12.10 32bit (the machine is 64bit, I think AMD), and if I check the C:\ drive with win7, it shows about 90 gb remaining. I'm new to ubuntu, and recently ubuntu said that only about 800 mb was left on the hard drive. How do I expand the amount of space ubuntu takes up on the hard drive so that it doesn't run out of space? -Many thanks in advance! (HowTos)