More than ones my PC was showing me an old website after changing the DNS records successfully. My Mac computer was already showing the new website, and if I looked up the DNS records on a web service like: intodns I could see that the DNS records were configured correctly.

If you ever run into this problem just reset the DNS table on your Windows computer. And that's how it's done:

Click on the Windows START button / open the system console (CMD) / and make the following entry:
ipconfig /flushdns

Done. Now your Windows computer will also show you the new website!