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If you are receving the error code 8100030d when signing into MSN Messenger or Windows Live Messenger, you can try out the following:

For MSN Messenger:
Click Start, click Run, type cmd , and then click OK to open the command prompt.

At the command prompt, type the following commands. After each command line, press ENTER:
cd\
cd %appdata%\microsoft
cd "msn messenger"
del /s listcache.dat

For Windows Live Messenger:

– For Windows XP:
Right click on the Start button and then click Explore to open Windows Explorer. On the Tools menu, click Folder Options. Click the View tab. In the Advanced settings list, select Show hidden files and folders under Hidden Files and Folders. Click to 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:\Documents and Settings\ Your Windows logon name \Contacts\ Your Messenger e-mail address
C:\Documents and Settings\ Your Windows logon name \Local Settings\Application Data\Microsoft\Windows Live Contacts\ Your Messenger email 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.

– 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.

Now try to sign in to your MSN Messenger or Windows Live Messenger again.

Credits: messenger-support.spaces.live.com