I agree with Gail. It sounds like the log backups are from a different database than the full/differential backups. If you look at the logical filenames using RESTORE FILELISTONLY, does it match the name of the database the full/differential backups point to?
http://msdn.microsoft.com/en-us/library/ms173778%28SQL.90%29.aspx
Joie Andrew
"Since 1982"