Square wave will power most computers just fine as the power supply will regulate it. It might draw a bit more power, buzz or get warmer but nothing to really worry about unless it is excessive. Cordless tools are a different story, some of the chargers are very cheap and basic and can toast on a square wave inverter. Usually you can find information about it in the manual where it will say you shouldn't use it on a inverter or not.
I've taken to running my PCs off DC/DC converters.
Mainly because I can't make up my mind what size and type of inverter is the best choice.
I have had big problems with these. The universal plugs don't fit well, and I generally shy away from anything using a conventional cigarette lighter plug on boats. Also, HUGE amount of RF noise even with a ferrite. With the DC/DC I could not use the laptop for wefax, and had to turn it off when using the SSB. I could do both with a cheap inverter and the stock laptop power cord. Currently looking for a good, relatively inexpensive inverter as my cheapo inverter has died.
Thanks, everyone. I've been skeptical of those DC/DC converters too, but am not sure if our $99 special inverter from Radio Shack is a whole lot better. Will do a little more searching then make VERY sure all our data is backed up before we try to run the laptop from it.