Installing a crack is both easy and legal. I haven't checked if there are any for AutoCAD, though.
That depends on where you are and the local laws.
Actually the local laws are irrelevant, it is wholly dependent on whether or not it is prohibited by the license agreement. You'll find some applications strictly prohibit the use of cracks, whereas others will not mention anything about them. if you look at the license on most games for instance it categorically states that circumventing the copy protection system is illegal and may be treated under copy protection laws, whereas some applications don't mention anything about applying what should really be termed as "unsanctioned binary patches" rather than simply cracks.
The distinction lies in whether or not you are a licensed user, and whether or not you are cracking a file to purely circumvent security systems which are in place to ensure you are a license holder, in order to use the software unlicensed or distribute the application. For games which use CD verification (securom and safedisc) to ensure the disc is with the user it is assumed you are unlicensed (until you prove otherwise) if you have a patched program executable.
In most instances if not all, software companies will ignore the fact you are using an unsanctioned binary patch in order to run their application, as long as you are in fact a valid license holder. However games may disable the valid license key for online gaming if you have a game binary which doesn't match one of their sanctioned check sums.
In some rare instances you'll find that the license becomes invalid after applying patches. In which case you are actually an unlicensed user with a cracked application so even having the disc doesn't matter as you're running an unlicensed copy of the software.
The simple fact is that the only real _law_ which can be infringed upon by using a patch is copy protection, and if you are a license holder and haven't produced illegal copies for distribution and it isn't prohibited in the license agreement then you're OK for the most part.
Hope this helps, K,