windows 7 32 to 64

hi im 15 and i bought windows 7 from PC world i asked about changing my 32 bit computer to a 64bit and the employee said to just stick the 64bit disk and it would work i put the disk in and it said it wouldnt allow me to install 64 bit which got me annoyed i put on 32bit and its alright but i have a quad core and a 32 bit only allows me to use 2 of those cores im only 15 and cant buy another copy of windows 7 is there anything you can do for me any help would be appreciated

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