Windows xp crash skype

My screen tilts often when I'm using Skype. If I'm using only Skype, this doesn't usually happen, Skype only works slowly most of the time. But if I have any other program running at the same time, it often happens that all activity on the screen stops: clock doesn't go forward, mouse doesn't move and even the off-button doesn't work. Only way to get things moving again is to take of the cord and the battery and start the laptop again.

I'm using Windows XP, computer is HP Mini. I have tried all kinds of virus programmes, just removed F-Secure to make the computer faster and changed to AVG free, but doesn't seem to help. What should/could I do?