It may or may not be valid depending on what the instructions are. Most ISP's give very comprehensive instructions on security on their web site and may occasionally send emails to there customers like NTL have done in the past, but no attachments.
These are the instructions NTL give:
Do you know what the instructions are? I personally wouldn't open any attechment, I don't who it's from, without proir agreement with the party sending me an attachment. Probably why I've never received a virus via email.