In the business world XP is almost certainly far MORE used than Vista and 7.
Also, I'm pretty much a noob when it comes to computers. I'm guessing that Windows 7 is the best OS. I'm considering upgrading my computer to Windows Vista, as it doesn't meet the requirements for Windows 7.
Go for 7. 7 is essentially what Vista should've been to start with, it's not packed full of new features but what is there (in my experience) runs far better on 7 than it does on Vista. There's really no point in upgrading to Vista these days.
I'm still running XP though, both at home and work and it's absolutely fine. If it does what you need it to then there's no need to run away from it just yet!