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The Windows 7 OEM license is non-transferable. You will need to buy a laptop with Windows 7 directly or Windows 8.1 Professional (so you can exhibit downgrade rights to Windows 7 Professional).
Regarding the processor the sockets don't match so CPU transfer is not possible.
You would have an i5-520M in the Vostro 3700:
Sockets supported BGA1288, PGA988
Comparing this with the Celeron n2830 which the Inspiron 3531 has:
Sockets supported FCBGA1170