Wrong digital signature for add-on installer

We have a machine on one site which is giving the following error when we try to install Enprise Job Costing add-on

Wrong digital signature for add-on installer

The site was upgraded from 2005 to 2007 last week. This one specific machine was formatted last night and the client (2007 pl 42) was reinstalled this morning. It however still gives this error when we try to install the Enprise add-on.

Please assist