If you haven't purchased a product key, you won't be able to activate it. That means, after 30 days, it'll go into "reduced functionality mode": the Aero display environment will stop working and you'll only be able to use a single IE window to purchase a product key from Microsoft.
At this stage, if you know how, you can use IE to open up explorer.exe and use your computer normally (for the most part), but many other features built into Windows won't work. This method is also tedious, because it logs you out without warning after a period of time.

So if the copy of Vista is pirated, you can pretty much forget getting it activated; you'll just have to reinstall every 30 days (depending on whether or not that release includes the Frankenbuild update).

My advice is to install Linux.