recommend two tables:

tblComments
CommentID (primary key)
ClientID (foreign key from tblClients)
Comments

tblCommentProjects
ComProjID (primary key)
CommentID (foreign key from tblComments)
ProjectID (foreign key from tblProjects)

if a comments record is about one project, then tblComments will have one
record, and tblCommentProjects will have one record linked to the
tblComments record. if a comments record is about five projects, then
tblComments will have one record, and tblCommentProjects will have five
records linked to the tblComments record.

you can use a standard mainform/subform setup for data entry, with the main
form bound to tblComments, and the subform bound to tblCommentProjects, with
a combo box in the subform bound to ProjectID, and the combo box RowSource
based on tblProjects.

your table names will probably be somewhat different, but the above should
give you the gist of the setup.

hth