Try this
this happens because you contact store is corrupted..
=►To delete the Messenger contact store, follow these steps.
★Note This procedure deletes all the contacts on the local computer. The contacts will then be downloaded from the server the next time that you sign in to Messenger.
=►Windows Vista
1.Show (unhide) hidden files and folders:
a.Click Start, and then click My Computer.
b.In the new window, click the Organize drop-down list, and then select the Folder and Search options check box.
c.In the Folder options window, click the View tab.
d.Make sure that the Show hidden files and folders check box is selected.
e.Click Ok, and then close the window.
2.Delete the contact store:
.Click Start, and then click Computer.
a.Locate the following folder:
C:\Users\Windows_log-on_name\Contacts\
b.Delete all the files that are in this folder.
c.Locate and open the following folder:
C:\Users\Windows_log-on_name\Local Settings\Application Data\Microsoft\Windows Live Contacts
d.Delete all the folders that are in this folder, including the hidden folders.
e.Close the window, and then try signing in to Windows Live Messenger.
1.Show (unhide) hidden files and folders:
a.Click Start, and then point to My Computer.
b.In the My Computer window, click Folder Options on the Tools menu.
c.In the Folder Options window, click the View tab.
d.Make sure that the Show hidden files and folders check box is selected.
e.Click Ok, and then close the window.
2.Delete the contact store:
.Click Start, and then click My Computer.
a.Locate the following folder:
C:\Documents and Settings\Windows_log-on_name\Contacts\Me
b.Delete all the files that are in this folder.
c.Locate the following folder:
C:\Documents and Settings\Windows_log-on_name\Local Settings\Application Data\Microsoft\Windows Live Contacts
d.Delete all the folders, including the hidden folders, that are in this folder.
e.Close the window, and then try signing in to Windows Live Messenger.