Virtualization is not the only solution to this problem, though it is the most stable. Though you may not respect Microsoft's work on Internet Explorer enough to buy it, you should respect your customer's decision to use it when doing web development. If you don't feel comfortable buying Windows or if your IE userbase is very small, there are other solutions. You've already found one option, the virtual machines that Microsoft makes available.
Another option is Wine Bottler, which can easily install versions of Internet Explorer as Mac apps through Wine.
If testing interaction is less of an issue, you can use IE NetRenderer to get a screenshot of your site in different versions of IE.