If it happens with IE and Firefox, try this
While browser add-ons can enhance your online experience, they can occasionally interfere or conflict with other software on your computer. Try starting Internet Explorer without add-ons to see if the problem goes away. Here's how:http://windows.microsoft.com/en-us/windows-vista/tips-for-solving-problems-with-internet-explorer
Click the Start button Picture of the Start button, click All Programs, click Accessories, click System Tools, and then click Internet Explorer (No Add-ons).
If disabling all add-ons solves the problem, you might want to disable all add-ons and then turn on add-ons only as you need them. This will allow you to figure out which add-on is causing the problem