SMTP junk mailing – any thoughts?

Situation,
A client I support has Windows 2K domain (sp4 on all) with AD, running a Exch2K server SP3. Until last week everything was running smooth then for some reason BT blocked SMTP relaying (now resolved), however outbound mail backed up so no notice was taken of the outbound queue. Having restored connectivity the queue remained huge and after examining outbound messages it seems a SPAM MAILER has intergrated itself into the SMTP service. Anyone else had this problem and how did you resolve it? Cant uninstall Exch or Service pack as it says component missing. Everything else works fine, the mailer is local to the server however no AV, trojan, or examination of folders turns up anything new.