The best bet if it's only for an accounts package it to use VMware Fusion (and a copy of Windows of course) with allows you to start Windows within OSX. It means you don't have long reboots just to open your accounts package.
Another option might be to look a Crossover, an application that allows you to open SOME Windows applications within OSX - have a look and see if it's compatible.
Also to save a bit of cash on buying Windows, have a look at the packages minimum operating system requirements (Win 2k, XP etc) and buy the cheapest one you can get on Ebay.