A bit of web searching suggests that certain versions of Skype seem to cause lock-ups with some Windows systems (particularly with XP). So the first thing to do should be to check that you're using the latest version. Skype 6.9.0.106 was only released a coupler of weeks ago so, if you've not already got that version, give it a try.
Otherwise try an earlier version. (Completely uninstall Skype first and then install the older version). Skype 6.3.0.107 should be your first choice. If that doesn't work, try Skype 6.0.0.120
http://www.oldapps.com/skype.php