Mac OSX logging itself out after SL upgrade

I recently upgraded my macbook from Leopard to Snow Leopard, and all seemed fine until i left my desk for a few minutes, only to come back to discover it had closed my open applications and was in the process of logging my account out.

Is this a common issue, and are there any ways to stop it? It's getting tiring having to reopen itunes/mail/safari everytime i need the toilet...

Thanks