You need to read the license terms carefully, some companies permit you to install a laptop copy in addition to the desktop on the understanding that you are the only user and will not use both machines at the same time. Others suppliers are less genenerous and will expect you to buy two distinct licences.
I use alwilss avast! 4 on both desktop and laptop, because it's good, free to home user and permits two copies. "The primary user of each computer on which the Software is installed or used may also install the Software on one home or
portable computer. However, the Software may not be used on the secondary computer by another person at the same time the Software on the primacy computer is being used."