I'm a newbie at the game and everytime I try to offer the ratpie to the stoat in the Crypt the game freezes and I get kicked out to the desktop..What can I do? Awfully frustrating! CD is a back-up from a friend...

The backup disc is very likely the problem. Other players have had similar results when using a non-original CD when conducting NPC interactions. And since these conversations/actions are integral to the game I'd recommend getting the original.