Change Content of Registration Email

Hi there

I am trying to change the content of the activation email a user gets upon registering on my site. I just can’t find the function that does it! Can anybody help me?

I’m running a multisite installation of wordpress and I have tried installing plugins to do this but they just don’t work, I thought it could be down to the fact that it was a multisite version.

Please can someone help me?!

Thanks