According to the user blogs I've looked at, this is a common problem Windows and Internet Explorer, where sometimes the set font size just happens to change for no obvious reason.
The answer for some has been to check the screen font size, which should be set to 100%.
In IE up to 7, go to the menu bar at the top, click view, then font size, and click 100% if it has changed from that.
In IE 8, the current percentage appears right down in the bottom right corner of the IE screen. If you click on that, it will show the various percentages you can click on.
If you have a mouse with a scroll wheel, you should also be able to hold down control (Ctrl) and use the scoll wheel to adjust the size (zoom in or out). Pics and graphics are adjusted too, not just text.
It can also be the case that some websites are optimising for newer computers with larger screens, and people with older, smaller screens need to use a larger font size and scroll the page.
Hope that helps.