Nigel,
Does it power up at all? If it attempts to boot up there are several options you can attempt before taking out the Hard Drive. Target Disk mode, Single user mode to repair possible permissions error, boot from install dvd to attempt a repair on OSX
If not you'll have to take out the hd and find someone who has an external SATA HD adapter to turn it into an external drive. Once you do that, copy over your user account and any place else you save files (obviously 😛 )