As Pullo said, it's the font that you have installed on your machine.
I had the same issue with Facebook, the version of Helvetica I had wasn't the 'official' version and it sent everything totally screwy and bold - just like what you describe.
Uninstall the version of Helvetica that you have and install a new font file.
The font stack used on here is "Helvetica Neue",Helvetica,Arial,sans-serif .
Have a look at your versions and reinstall if you can