You should be able to follow these steps and get things fixed:

1. Determine what version of Messenger is installed on the computer, and then upgrade Messenger if it is required:

1.a. Click here to determine the most recent versions of Messenger for each operating system. If you are not running the latest version for your operating system, please install the most updated version.

2. Delete the cache files for Messenger. Follow the steps below based on which version of Messenger you are using:

2.b. For Windows Live Messenger:

2.b.ii. For Windows Vista

• Click Start and then click Explore to open Windows Explorer. On the Organize menu, click Folder and Search Options. Click the View tab. In the Advanced settings list, select Show hidden files and folders under Hidden Files and Folders. Clear the Hide extensions for known file types check box. Click OK to close the Folder Options window.
• Delete the Contacts Cache folder. To do so, right-click Start, and then click Explore to open the Windows Explorer window. In the Windows Explorer window, locate the folders below and delete all the files and folders inside them.

C:\Users\ Your Windows logon name \Contacts\ Your Messenger e-mail address
C:\Users\ Your Windows logon name \AppData\Local\Microsoft\Windows Live Contacts\ Your Messenger e-mail address


Note: Windows Logon name is the logon name that you use to sign in to windows. Your Messenger email address is the address that you use to sign in to Messenger.

3. Try to sign in to Messenger again.



You can find all this info, and info for addressing any AVG issues that also might be causing this problem here:

Error: 8100030d - Windows Live

Cheers,