*OEM = Original Equipment Manufacturer (used by computer system builders).
*The OEM version needs to be install on a new hard disk (not one with an operating system already on), or you could wipe
clean the operating system disk before installing.
* It will bind to the first computer it is installed on and cannot be transferred to a different computer at a later date.
* There will be no free technical support from Microsoft as it will be provided by the system builder (you).
Window 7 (W7) comes in 32bit and 64bit favours. It also comes in several versions Home Basic, Home Premium, Professional, Enterprise, and Ultimate. Most home users get by with the Home Basic edition.
A CPU needs to have 64bit architecture to run W7 64bit.
A CPU with 32bit architecture can ONLY run W7 32bit.
A CPU with 64bit architecture can run either W7 32bit or W7 64bit.
You can usually tell what architecture your CPU has from the BIOS.
W7 32bit can only address about 3.5Gb of RAM.
W7 64bit can address much more than 3.5Gb of RAM, depending on the version (e.g. basic (8Gb), premium (16Gb), professional (192Gb), etc.)
Most 32bit programs will run on W7 64bit, however notable exceptions are device drivers (e.g. for printers), these will need to be 64bit to run.
My install went ok, most drivers are downloaded automatically. The only problem I had was a PCI 4USB card that didn't get detected. This was solved by switching the PC off and removing the power cord for a few minutes (power is still supplied to the main board even when PC is off). Upon reboot the card was detected fine.
Oh and do be prepared to have W7 download and install a ton of automatic updates, mine had to go through 192Mb!