Solution 1:

Internet Explorer (under version 9) does not natively support rounded corners.

There's an amazing script that will magically add it for you: CSS3 PIE.

I've used it a lot of times, with amazing results.

Solution 2:

Rounded corners in IE8

Internet Explorer 8 (and earlier versions) doesn't support rounded corners, however there are few other solutions you may consider:

  • Use Rounded Corners Images instead (this generator is a good resource)

  • Use a jQuery Corner plugin from here

  • Use a very good script called CSS3 PIE from here (Pro's & Con's here)

  • Checkout CSS Juice from here

  • Another good script is IE-CSS3 from here

Even though CSS PIE is the most popular solution, I'm suggesting you to review all other solutions and choose what works best for your needs.

Hope it was useful. Good Luck!