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#11
If the windows 7 DVD you have now is 64, make an ISO file of it. Then edit it and delete the ei.cfg file in the sources folder. Now burn the DVD and the new disk will let you install any version you want. Leave the product code blank and you can legally use it as a 30 day trial. If you don't have a 64 bit version borrow one or ask someone to burn you a copy of theirs. If no product codes change hands I don't see a problem.
These tutorials may help:
Windows 7 USB/DVD Download Tool[2]=General Tips
Windows 7 Universal Installation Disc - Create[2]=General Tips