Various browsers render text a little different. In this case, the text in the buttons is shown larger in IE than in other browsers, even though it’s both set at 15px. However, the fact that the font family is set to “serif” isn’t helping (not a standard font so will revert to the very basic Times New Roman).
I’d say there’s 2 ways of fixing this.
1. Use a conditional style sheet for IE, where the font is set to 13 pixels.
2. Give each button a specific fixed width in pixels.
I personally prefer the second one, as it will eliminate the whole browser incompatibility thing (not just IE) and so the buttons will always be the exact same width in every browser.