This can be done easily by:
- clicking the START button
- clicking ALL PROGRAMS
- scrolling down in the list of programs until you locate the browser (IE, Chrome, FF, etc)
- keep the SHIFT key pressed
- right-click on the browser app
- click on RUN AS DIFFERENT USER
- enter the domain, username and password of the new account
- the browser will be opened under the new account
With this tip, it is possible to have the same site opened in different instances of the browser, each instance using a different account.