Unauthorised email sent from my colleague's account.

[nihil]

Well,

It would not do any harm to do a full safe mode scan for malware?

There have been some nasties of late that fake a returned e-mail as their vector. User sees this and opens it...........................oooooooooops!

just a thought

[zENGER]

Since most of these virii grab addresses from the address book, getting a new email account won't really do anything. As soon as he lets all of his collegues know his knew address and they add him to their address books again, his knew address will most likely be spoofed. I agree with Milner that this is more or less to be bulked with spam and ignored unless there is a real compromise.