Developing websites requires testing them on most browsers, mainly to check CSS interpretation which can differ from one browser to another.
You can either install those necessary browsers on your local machine or have your website tested online.
I usually develop website on Mozilla Firefox or Iron Browser (faster) and have experienced most testing must take place on Internet Explorer.
Tredosoft provides a multiple install of main IE versions from 3.0 to 6.0 (for Windows XP). Your Windows partition should host IE 7.0 or 8.0.
You may also test your website online : Browsershots provides an online test for major cross-platform browsers (Linux, Windows, Mac,…).