According to Microsoft:

Through the online provider, the Sign-in Assistant helps a Windows 7 application running on one computer identify and communicate with other computers that are associated with the same Windows Live ID. Just by linking your Windows Live ID to the user account on each Windows 7 computer you own, you enable those computers to identify each other and work together more easily than has been possible in the past.

Here’s the quick how-to:
1. Open Control Panel and click on “User Accounts“. Now click on “Link online IDs” link given in left sidebar.

2. Now click on “Add an online ID provider” link given at the bottom.

3. It’ll open a web page in your browser. You’ll need to click on the “Windows Live” logo present in the middle.

4. It’ll open a new web page where you can download the “Windows Live ID Sign-in Assistant” which will install the required “Windows Live ID online provider”.

Alternative:
You can click here to download the Windows Live ID Sign-in Assistant.

Notes From Microsoft:
If you have an online account, such as an e‑mail account, you can link that account with your Windows user account. Linking these accounts provides the following benefits:
•People can share files with you on a homegroup using your online ID instead of having to create a Windows user account for you on their computer.
•You can use your online ID to access your information on other computers on a network, such as accessing files on a home computer from your work computer.