An email was recieved announcing that it will be required for all MSN beta participants to upgrade to the MSN Beta 7. This update is mandatory, and after February 5th, anyone running pre Beta 7 will not be able to sign in to MSN. To check which version of MSN Beta you are running, navigate to the Help & Settings menu on top and select About MSN.

Beta 1 - Version: 9.00.0008.2602
Beta 2 - Version: 9.00.0009.1802
Beta 3 - Version: 9.00.0010.1601
Beta 4 - Version: 9.00.0010.2800
Beta 5 - Version: 9.00.0011.0500
Beta 6 - Version: 9.00.0011.1200
Beta 7 - Version 9.00.0013.2101

Beta 6 will automatically update to 7 in the background when you sign in to MSN.

For Beta 1-5, after signing in to MSN on or after February 5th, a dialog box will appear asking you to update. Click Install Update Now (or Update Later). MSN will close, update and then restart.

Beta 7 allows support for Windows XP Service Pack 2.

News source: Winbeta