I know next to nuthin' 'bout server setup but I found a couple pages that may answer your question. [windows2000faq.com...] [windowsitlibrary.com...] I also invited our resident M$ expert to stop by and add his comments.
In the Internet Services Manager, go to your web site. Then right click on it and select properties. On the Web Site tab of the properties dialog, there is the Connections section. In there, you can change the number of connections from Unlimited to Limited To, and specify a number.
If you are not the ISP, you will need to get your ISP to do this for you.
You may also want to check out the Performance tab, where you can limit the bandwidth throttling, and the processing throttling of the web site. These allow you to limit the server to say 1024 KB/Second, or 10% of the CPU time.