As stated you'll need to do a clean install.. UNLESS you want to setup a dual boot configuration. You could keep your existing installation and just install a 64 bit version of Windows 7 on a seperate partition or a seperate HDD.. That way you could keep XP around for those few apps that just won't work on 64 bit yet..
Having a 32 bit and 64 bit OS installed on either two seperate HDD's or the same HDD but on seperate partitions will work fine..
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