Cheap laptops tend to be just that: cheap. Toshiba doesn't have a good reliability reputation, same for Dell. Asus I personally had one that came defective, though I hear that is the exception rather than the rule, and it's probably the best budget laptop now.
Business class notebooks are way better than these "consumer" class notebooks. Thus the reason why Apple (used to) have macbook and macbook "pro." Materials and build-quality are better. Go with business class, you'll be much happier even though the price is higher. Dell and HP both have business class laptops that are probably $1000+, but for various reasons, I don't like them.
For PC, I highly recommend Lenovo Thinkpads. Thinkpads used to be made by IBM, but they sold their computer division to Chinese company Lenovo. These were the only laptops certified for use in space (you know, in case you start rocketing off mid-exam session). However, the quality doesn't seem to have dropped. The keyboard is extremely reliable and not too springy or cheap-feeling. The components tend to be top-notch, not re-branded cheap components like Dell does. Thinkpads AFAIK all come with sealed spill-resistent keyboards (drains out the bottom) and with the smaller models, the battery lasts forever (When my X201 was new 1.5 years ago, I'd get 7-8 hours with wifi on, 14/15 screen brightness). Lenovo has periodic discounts, you can get one for ~$1000.
Now, the debate between Windows vs. OSX is a whole other story...law schools seem to be becoming more Apple-friendly (in terms of the exam software AFAIK) so it probably doesn't matter.
Lenovo Thinkpad = good
Panasonic toughbook = great (they can take pretty much any abuse, and are extremely durable; too bad they're expensive...)
Also, you need not choose windows or OSX; use Linux/BSD.
I must note that I've not had any trouble with dell's in the past, but Lenovo Thinkpads/Panasonic Toughbooks are well known for their durability. Over their lifetime (so long as you take care of them), they're worth the investment.