Generally microsoft doesnt believe in "trial" versions, but if you are in school often you ccan get 120 day liscences for some software. There are also the older beta Versions, im not sure how long till they expire but it could be enough to get your feet wet.
 just checked , the betas have expired.
You can, however, use the actual install cd until activation expires, without using a cd key, or if you have an MSDN licence, the same thing can be downloaded. Either of these versions will have to be purchased within that 30 days though, or will become invalid.