Is your preferred browser suddenly displaying all the basic fonts in an unclear and garbled way? Before starting to pull your hair off, it might be that something got in the way and some Windows preference has been changed.
What happened to me once, is that Firefox, Chrome and Safari started displaying system fonts strangely, while Internet Explorer was rendering the text in the correct way.
I remember that I started searching for a solution, and, at first I thought it was something related to my system font files that got corrupted. But it wasn't that.
I noticed that other programs and applications were displaying the menus and text in a strange way.
I decided to rebuild the font system folder. Nothing changed...
Well, to make a long story short, it was the Clear Type option.
In Windows 7, there's a control panel section that's taking care of Clear Type fonts: ClearType tuning.
In Windows XP and Vista, we need to download ClearType Tuner from Microsoft.
If for some reason, the clear type option has been turned off, the result will be that all the fonts in your applications will be in some way less clear (sorry for the pun). And even your browser will display system fonts in that way.
Just turn ClearType on again and everything will be fine.
More info here.