is it possible to share my desktop pc internet to my mobile phone?
im using HTC Hero and a wired internet connection on my desktop pc
the hero's wired tethering is one-way only i.e. letting the PC share the phone's connection. Certainly that's all you can do without jailbreaking
if you attached a wi-fi adapter to your PC, you might be able to set up an ad-hoc wireless network and share that... but in that case, you're just as well off just buying a wireless access point
It appears some people have attempted to do just as you describe... with little success. Here's another. Sorry. It looks like it's not yet possible with Android (the operating system of HTC Hero).
If the desktop has wifi and Widows 7, you can try using something like Conectify to share the wired connection over wifi. I've only used this to share a wifi connection (sort of like a repeater really) but should work the same.
On Vista, I believe it's possible to share a wifi connection over wired but can't find anything the other way around as required in this case.