YEs, you can restore just the primary filegroup from the backup, even if it's the full database backup.
Look up the RESTORE DATABASE syntax, you're looking for the FILEGROUP = option, and you'll want to specify PRIMARY as the filegroup to restore.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability