There is some differences with Windows XP when you change the computer name in Windows 7.
Go to Start, select Computer and right click Properties:
Click Change Settings in properties windows:
Please note the upper space textbox is not for computer name, it is for computer description, If you want to change computer name, please read bottom section which is for rename the computer or its domain or workgroup. Click Change… button
Now you can see the place to change your computer name, and also change domain and workgroup names.
After you changed names, click OK button , and system will ask you to reboot system. You have to reboot.