I have recently installed additional Helvetica font that was required by some .PSD I needed.
After I while, I have noticed that font has changed on both IE and Chrome browsers on some pages. Because I didn't like the change I tried to delete the font from Control Panel\Appearance and Personalization\Fonts
Unfortunately I wasn't able to delete the font and did receive this message:
Delete Font: Helvetica cannot be deleted because it's in use.
I have tried by closing all programs but I just couldn't delete it.