There are various bits of Windows and Mac software that let you do this but I assume you don’t want to install anything on the work PC?
I use an app called AcePlayer. Handles pretty much all movies and audio formats.
It will happily connect to any DLNA media servers, FTP, SAMBA, or HTTP servers you give it and let you transfer files from there.
Or it can appear as a web page which you can access from your PC’s browser and drag files onto it.
Downsides: you obviously need to have your iPhone on the same wifi as your PC. And downloaded songs/movies are stored with the app, not added to the main iTunes library.
If you have any trouble using it then consult http://ranysoft.wordpress.com/