You can reuse one email address multiple times in order to register Avast on multiple machines. With Avast Free, you can register online and it should be immediate. There is no need to receive an email confirming the registration. The link Asyn provided contains the details, and the information is there if you read it carefully.
AMD A6-5350M APU with Radeon HD Graphics, 8.0GB RAM, Win7 Pro SP1 64bit, IE11
i7-3610QM 2.3GHZ, 8.0GB Ram, Nvidia GeForce GT 630M 2GB, Win7 Pro SP1 64bit, IE 11
Common to both: AIS 2015.10.4.2233, WinPatrol Plus, SpywareBlaster 5.2, Opera 12.17, Firefox 41.0.1, MBam Free, MCShield, CCleaner