Port 80 proxies

It seems that only port 8080 proxies work with my pc. Why?

I have tried using countless different port 80 proxies that are suppsosed to be anonymous but they spill my real i.p.
If I keep the same internet settings and use a port 8080 proxy that conceals my i.p. and then replace it with a port 80 proxy, my i.p. gets spilled!

Is there something I am doing wrong?

Thanks,
C-zer