Before answering this question you need to know what are the different IIS version is available in different OS. Below is the list of IIS version with different Operating system.

Windows Server 2008 - Windows Vista - Home Premium/ Ultimate - IIS 7.0

Windows Server 2003 - IIS 6.0

Windows XP Professional - IIS 5.1

Now based on your working experience you can say that you have worked on IIS 5.1 and 6.0 or only IIS 7. Etc.

Now, the next question that can asked after answering this question is “what is the difference between them ? ” – Well I will come with this later.