From what you've written above "from any PC" I'm assuming you want to be able to add/manage your multimedia from more than one PC / iTunes instance right?
If not, and you are only using the one PC / iTunes, then it should be really easy for you.
However if you are doing this on multiple computers, is this the message you are getting?...
"The iPhone 'name' is synced with another iTunes library. Do you want to erase this iPhone and sync with this iTunes library? An iPhone can be synced with only one iTunes library at a time. Erasing and syncing replaces the contents of this iPhone wtih the contents of this iTunes library."
If it is, then it is just what it says, you can only manage the music/video library from one iTunes. I created the below post earlier but haven't had any replies yet. I do hope I'm wrong, but I'm pretty sure it can't be done using iTunes.
http://www.iphoneforums.net/forum/iphone-4-help-15/two-instances-itunes-6884/
The way I do this, is have my home computer 'sync'd' with my iPhone, and when I want to add/remove music from my iPhone on another computer, such as my laptop or work computer. I just use foobar, which is an iPod / iPhone management / media player. There are others such as Winamp and amarok etc. However I can only vouch for amarok (linux) and foobar as definitely working properly.
Good luck and I hope you get it all sorted. If anything I've said is incorrect please feel free to correct me, I am new to iTunes but from what I gather this is how it is.
Craig