Yes it can, however the benefit will probably be minimal. It will be a great amount of help for a data warehouse, but for OLTP applications the data will be so fragmented throughout the file, that defragmenting the file on the drive will only provide a minimal amount of assistance.
More is gained by making sure that the database file doesn’t get fragmented in the first place.
Mr. Denny is quite correct, but the real point is that tuning databases is almost a career in itself. Just doing a search on www.amazon.com searching for “sql server tuning” brought up many matches. Sorry I can’t provide any real help – other than to point out that database performance tuning is not an exercise for the faint of heart or inexperienced.
Any kinds or types of defragmentation like registry and hard disk could significantly increase the overall performance of the computer system including its applications installed like SQL Server. This should actually be done at least once a month depending on the activities of your hard disk drive.
Defragmenting hard disks could definitely improve SQL Server performance. Remember that SQL contains and stores files, therefore defragging the entire hard disk drive could not only improve the SQL performance but it could also improve the entire performance of the computer sytem.