The iphone can sync to only one computer at a time. When you connected it to another computer it switched its alliegence to the new one. You will have to sync with your computer first, then drag and drop should work. But it's a poor way to manage music; it's much easier to create an iPhone playlist, copy all the music you want on the iPhone to the playlist, then sync that playlist to the iPhone. If you have a lot of tracks (I have 3500 on my phone) create several lists to sync.
Thanks for the quick reply mate. I connected my iPhone to my PC again, sync'ed and then tried manually dragging dropping music but it's still having none of it.
I tried your suggestion in creating a new iPhone playlist. I drag and dropped the music from my Library to the 'Playlist' section in the left hand panel and it created a new playlist. When i tried dragging and dropping this playlist (from the left hand panel) onto my iPhone the same thing happens as before. The drag icon still a circle with a line through it not allowing my to drag and drop that either.
Sorry for being pain, any other suggestions? I'd still kinda prefer to manually drag and drop music as i did before as i didn't have any problems before.