We regularly use a lap-top using either an inverter (don't know if is a 'quasi-sine wave' one - it cost about £30 from Maplins) or of late a 12-19volt cigar socket plug. I gather that these are not the 'right' way to proceed, but we've never had any problems whatsoever
Mine (HP) will run (but wont charge the battery) on a straight 12v DC feed. However I prefer to use a cheap 12v to multivolt (15 to 24 in numerous steps) upconverter to isolate any nasty spikes on the boat supply. I got one for about £8 including post from an EBAY retailer in Guernsey.
Better to use one of those travel gadgets which boost 12V to whatever your PC needs (adjustable Up to over 20V I believe). Should cost no more than 25 Squids. Much better
power consumption and no problem with wrong wave form also. Computershops or electronic shops should carry them.