slow down geezer!
first thing is make a list of everything in your system and set yourself a PSU power.
Then mulitply this by 1.5 and that is your inverter.
As for speakers.....are you wanting to use some home PC 5.1 speaker sin your car? So you want to run them off an inverter too?
As for the laptop screen - don't bother - it will cost you more than buying a VGA screen as you will have to get some kind of controller - and you shall see why.
Also dont forget you will need some kind of startup/shutdown controller and to be honest i would reccomend a DC-DC solution over an inverter. If you are careful and dont get too greedy with the P4 spec and dont use lots of 12V devices (5.25" optical drives, multiple 3.5" HDDs etc etc) then something like the Opus 150W will do you or you can wait for the new higher power models while you get on with building your system.