Did you even noticed sometime few website work on PC and some others not. Restarting your PC may solve this issue and all website open as usual. This problem may come due to invalid DNS cache and can be easily solve if you flush your system cache. I am showing you exactly how you can flush dns cache yourself in windows 7. The steps should be same for Windows Vista.
Click on Windows7 start button and click on All Programs
Click on Accessories then right click on Command Prompt and select Run as Administrator. You may receive following warning Do you want to allow following program to make changes to your computer. Click on Yes button to open command prompt window.
Type command ipconfig /flushdns in the command prompt window and hit enter. You will receive confirmation message as shown below in the picture.