helvetica font on Windows

I am really frustrated that even on your website you use Helvetica  
fonts rather than say Arial – a proper web safe font.  Helvetica does  
not render even remotely well on my PC using firefox or chrome, I run  
windows XP.  I have Helvetica installed on my computer but it does not  
render for a lot of people using PC’s, so why is it in use so widely  
and why do you use it on your website? Why is Arial not used as a  
proper for screen designed font?