If the system won't boot up at all then the motherboard might indeed be fried, but if your looking to put together a good desktop system you'll find they are much cheaper and much easier to upgrade than laptops.

If your going to take the hard drive out of your laptop at some point for use with your desktop, I'd recommend purchasing an external USB drive box for it.  I've got an old 10 gig from a laptop that I put into an external USB drive box, and now I can just connect it to the USB port whenever I need a bit of space to transfer files.

The box itself cost me about $12 USD, and the drive was one I had laying around so the expense was negligible, and now I have the ability to transfer even lots of very large files easily, the box itself is small enough to fit in my shirt pocket.  Generally speaking desktop harddrives are dirt cheap by comparison to laptop harddrives, my current desktop system has 520 gigs of harddrive space so 10 gig extra by comparison is a waste of a perfectly good drive bay, but 10 gigs fo portable space can be a real godsend.  

One other thing you might want to consider would be putting the laptop you have up for sale for parts - if it's an expensive laptop you'd be surprised how much you can get for some of the various parts in the system.   Granted, you're not going to recoup all of your losses, but things like laptop screens sell fairly well on Ebay because they are hard to find elsewhere for most laptops and the manufacturers want an exhorbitant amount of money for replacement parts if you deal with them directly.   They know they have you over a barrel and price accordingly.