Verdana on Win XP is another one you can use that looks close. You have to understand there's just some fonts that came standard on the different platforms, and now people use them a lot, but they don't come on the opposite platform unless you buy them. Helvetica is one of them. It's not just a single font, there's actually a whole set of helvetica fonts carefully drawn to different weights and sizes, so there's actually 20 or so "helvetica's." Thats probably why its $30 for the font. Anyways, about your problem, unless you can find a pirated version that someone bought online, you're kind of stuck. I actually wouldn't reccommend going looking for a pirated version either. Lets face it, you're on Win XP and font files are easily disguised as something else. Also, many people don't realize it, but you're supposed to pay for fonts just like you pay for programs, they're not something you're supposed to pass around freely. On the other hand, almost everyone does.
I think your best bet would be to find a friend on a mac who has the old old program "fontographer." Its a font creation tool, and they can change helvetica into a PC version, its kind of a handy program actually, when you're working in a cross platform office. Maybe someone here has it and can send you a PC version