Ask the manufacturer (who designed it to connect directly to the parallel port with no isolation) and I believe he'll tell you that he hasn't heard of anyone's PC being damaged through a Xylotex. (Think I read that somewhere)
Ask someone who sells an isolated board and he'll tell you why you should use one. the easiest set up I've seen is from www.candcnc.com Plugs in between the Xylotex and parallel port.
Keep in mind that if you connect your Xylotex directly to the parallel port, and miswire anything, all bets are off in regards to your PC's safety.