You will need to "activate" the Reader program on every computer or device that you intend to use for reading your ebooks. Therefore, if you have a PC at home, a laptop for work and a handheld device, you will need to activate MS Reader once on each machine. The link to activate the program (once it's installed on the machine) is: http://das.microsoft.com/activate/en-us/default.asp
If you do not yet have a Microsoft Passport, you can create one when activating the software. If you do have a Passport, simply sign in and proceed with the activation process.
Note: if you have a Pocket PC attached to your PC, you will have two options on the activation page:
Please note: you must have access to Admin privileges on your windows based PC to allow Active X controls which are required to activate MS Reader. Corporate PCs may not allow you to login as Administrator.