You will need to upgrade to Windows 7 Ultimate for this capability. Only Windows 7 Ultimate includes Multilingual User Interface (Language Pack) support. You can then install the English Language Pack from under Regional and Language settings in Control Panel.
You can do an Anytime Upgrade to Windows 7 Ultimate which will preserve your personal files and settings:
Learn more:
http://windows.microsoft.com/en-us/windows7/products/features/windows-anytime-upgrade
Install Language Pack in Windows 7 Ultimate
1. Start -> All Programs --> Windows Update --> Optional Updates.
2. Open optional Updates and go to the section Windows 7 Language Packs.
3. Tick the Language Pack you want and hit OK
4. Proceed with Install Updates
5. Depiction of Windows 7 downloading a Language Pack
6. Automatic install initiated after download
How to change the Windows 7 Display Language:
1. Go to Start -> Control Panel -> Clock, Language, and Region / Change the display language
2. Switch the display language in the Choose a display language dropdown menu.
3. Click OK
4. Log off for the changes to take effect (This will affect only the user profile under which you are currently logged on).