Drop the foreign key first, then drop the index. I suspect you won't be able to recreate the foreign key unless the index is recreated.
Why do you need to drop them? If you're rebuilding the indexes, what's wrong with DBCC DBREINDEX?
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