Hi mate...

eBay aint changed anything, far as I can see, and its basically your browser that decides the font used in Windows.

Couple of things .. There is a trick you can do to change the font size quickly on web pages, especially if you have a scroll wheel. Press the Ctrl tab and hold down, then rotate the wheel. This should change all html text size. At non-standard settings, it looks weird, like you descibed.
Also check the font set in Internet options front tab of settings .. Default is T N Roman.
Also in Display/Settings/Advanced, you can check the settings there.
It's unlikely that a worm/virus will do what you describe ; )