I had the "joy" of working on a new laptop that came with 32-bit instead of 64-bit 7 Professional. 2.5GB out of 4GB usable RAM + no XP mode: I wasn't going to allow that.
1. Obtain a 4GB flash drive & any x64 disc of Windows 7.
2a. Install MagicDisc & follow this guide to copy over the files to the USB.
2b. Install & use the DVD to USB tool.
3. Edit sources/ei.cfg on the USB drive with a Notepad variant: it should be Retail + the version of 7 you have the key for on the computer.
4. Boot the system with the USB drive & install Windows 7 as normal, using the key that came with the computer.
5. When you are satisfied the system is stable, activate Windows: you'll more than likely have to call the Microsoft automated activation.
Reference: How to install any version or sku of Windows 7