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#11
If windows was already preinstalled, it was already either 32 bit/64 bit so when and how did it give you the option to choose 32 bit/64 bit? Stop playing games, no OEM gives those kind of options. And if it is some unknown, boutique OEM, I'd like to know their name.
As far as installing 64 bit goes, locate a 64 bit disk, install the same edition that came preinstalled and activate with the sticker key. You'll have to install all drivers and apps afresh.