An interesting article on why us Windows users were so lucky to end up with the Arial font pre-installed on our systems rather than Helvetica.
Short answer: Microsoft were cheapskates:
When Microsoft made TrueType the standard font format for Windows 3.1, they opted to go with Arial rather than Helvetica, probably because it was cheaper and they knew most people wouldn't know (or even care about) the difference. Apple also standardized on TrueType at the same time, but went with Helvetica, not Arial, and paid Linotype's license fee. Of course, Windows 3.1 was a big hit. Thus, Arial is now everywhere, a side effect of Windows' success, born out of the desire to avoid paying license fees.