Dell Latitude D620 Laptop - windows 7 64 bit

There are variants of processor available in the Latitude Dx20 series. To determine what one you have use the Intel Processor Identification Utility:

http://www.intel.com/support/processors/tools/piu/sb/CS-014921.htm

The processors in this model are typically one of:

Intel® Core™ Solo processor T1400 (1.86GHz)
Intel® Core™ Duo processor T2300E (1.66GHz) T2400 (1.83GHz) T2500 (2.0 GHz) T2600 (2.16GHz)
Intel® Core™ 2 Duo processor T7200 (2.00GHz), T7400 (2.13GHz), T7600 (2.66GHz)

http://www.dell.com/downloads/global/products/latit/en/spec_latit_d620_en.pdf

To install the 64 Bit version of Windows you will need to upgrade the processor to an Intel T7200, T7400 or T7600 which have a 64 Bit instruction set and hence will support Windows  7/8.1 64 Bit.

Note regardless if this is done or not the chipset on the machine limits the usable RAM to 3.24 GB so if you are upgrading to address more RAM then you should note that on this particular model the 64 Bit version of Windows cannot access more RAM.

I upgraded the processor in a Dell Latitude D820 from a T2400 to a T7200 and the Dell Wireless Bluetooth 360 (Toshiba) to a Dell Wireless 355 (Broadcom). This allowed me to install Windows 8.1 64 Bit and get it running correctly. Windows 7 performs slightly better on this model.

You will see the instruction set is only 32 Bit:

http://ark.intel.com/products/27236/Intel-Core-Duo-Processor-T2500-2M-Cache-2_00-GHz-667-MHz-FSB

The processor replacement does not require a new motherboard however to replace it you pretty much have to dismantle and reassemble the entire system to do so. If you are comfortable doing so you can refer to the service manual for details.

A T7200 can usually be found for cheap on eBay. A T7400 and T7600 are usually much more expensive to the point that the upgrades not worth it but you might get lucky. You will also need to buy some thermal paste.

Installation of Windows 7 32 Bit is far easier. You can use the same product key with 32 Bit media which saves you from going out and buying Windows 7 again. Typically I would recommend Digital River which are an official partner for Microsoft and host Windows 7 installation .isos. However their servers for Windows 7 seem to be broken these few days. See here which links to Heidoc.net for details:

http://dellwindowsreinstallationguide.com/download-microsoft-windows-and-office/download-microsoft-windows/windows-7-sp1-iso-download/