One user Sending out mass spam

Server 2003 running exchange 2003
the Client has XP running office 2007

I have one Client sending out about 10 spam messages an hour automatically. No one else that uses that server is having this issue.
I assumed he had some sort of virus on his PC so I formatted it but even after the format the spam is still sending out

I did multiple scans on the server with malware bytes, super anti spy ware, mcafee and the cleaner 2011 and found 0 infections on the server

has anyone had this experienced before with a spam sending virus only effecting one user? How Do I deal with this if multiple scanners are not picking up any infections?

Thanks
Ryan