Like others say, If you've not shot on the cards since deleteing you should easliy be able to recover all of the images. You can do the same with images on your hard drive, provided they haven't been over written - if as you say, your computer simply "crashed" after downloading, the data sectors containing the images on the hard disk should still be mostly in tact - it's more likely that the "address" or "index" of the image files that's likely to have been lost, hence you not being able to see them.
I did a blog post a while back about recovering lost data: This piece of software has come in useful on many occasions now - it's free, too:
http://www.martynlewisphotography.com/blog/post.aspx?id=3ea743e3-5e80-453e-9193-dcdade78e670
With that software, I managed to recover every single one of my images from the day, that had accidentally been deleted. In the process, it also recovered much older photos that I'd long since deleted, of a wedding from the previous year: I'd been using the card regularly too, so it would seem that certain parts of the memory card hadn't been touched in months.
(Incidentally, i did the same on a large capacity card that a friend had borrowed for his holiday. Recovering some unrelated images a couple of months later, In file recovery mode, and not just "image recovery mode", it brought back all sorts of interesting "home videos" he and his Mrs had seemingly engaged in... Needless to say, i was discrete and professional about it. and he often buys me a pint or two, for my silence! I haven't looked at his innocent looking other half in the same way since! So just be careful what you recover for friends and family!)
