Why do you want Windows 7 early? Windows 7 is a very good/more reliable replacement for Windows Vista and solves many of its problems.
The reason you do not want to replace your existing operating system: The RC will expire June 1, 2010, and the bi-hourly shutdowns will begin on March 1, 2010. In both cases, you’ll need to rebuild your test PC to replace the OS and reinstall all your programs and data.

System Requirements:
• 1 GHz processor (32- or 64-bit)
• 1 GB of RAM (32-bit); 2 GB of RAM (64-bit)
• 16 GB of available disk space (32-bit); 20 GB of available disk space (64-bit)
• DirectX 9 graphics device with WDDM 1.0 or higher driver

Step 1: Download the OS

http://www.microsoft.com/windows/windows-7/download.aspx
The download will be available until July 2009.

Step 2: Burn to a DVD

If you do not already have a software that can burn an iso onto a bootable dvd then download and install Active@ ISO Burner http://www.ntfs.com/iso-burning.htm
After you install the software open the program, put the dvd in, load the iso, and burn it to the dvd.

Step 3: Install a Partition Manager

If you do not want to replace your existing windows operating system completely with windows 7 then you need to download and install the Easeus Partition Master Home Edition. http://www.partition-tool.com/
Use the partion manager to resize your current petition and to create a new partition at a size of your choosing as long as it is bigger than 20240Mb (20 Gb). After you have created your new partition, you will need to reboot your computer so that the changes can take place.

Step 4: Install the OS

Now that you've created the new partition you can load the DVD and install Windows 7. Make sure you choose the new partition you've just created to install Windows 7 on so you do not delete your old operating system and files.

Step 5: Enjoy Windows 7