Waterproofing a screen is no big deal and you could use ceramics(or something else) to protect the cpu.
Yes very expensive you would need to be working for James Bond
I bet with the right materials you could build a curcuit board and chips that would work just fine in a salt water enviroment then you could use the water itself as the coolent.
I dont think heat will be the problem if you want to...
The best security for a laptop is to never leave it where it can be screwed with.
Anything else is a waste if you dont keep a good eye on your system.
Like others have said if you give some one physical access to your laptop then...
If you have the OS install disks you can just reformat the drive and do a totally clean install from the disks.
Just make sure to delete the old partitions and create you own. Then do the install and you will have a clean...