If you have activated the automatical Windows updates on your PC, it's possible that the Microsoft SQL Server can't be installed cause of a newer version of the MSXML 6.0 Parser.

In this case, you can uninstall the "MSXML 6.0 Parser" in your installed program list, and try the installation again.

In some cases, the MSXML Parser can't be uninstalled.
For such cases, Microsoft published a "CleanUp Utility" on there website (http://support.microsoft.com/kb/968749).

Since this tool ("MSICUU2.exe") is no longer available on the Microsoft support website (http://support.microsoft.com/kb/290301) you can try to get the tool by internet research - some forums still have a copy of it.

best regards