There are a few things which you will want to look at. First make sure that you have enough RAM for the database to use. Without enough RAM in the SQL Server you’ll begin putting extra load on the disks because the same data is being pulled into the buffer cache from the disk over and over.
The second thing to check is that you don’t need to add any additional indexes. If your queries are doing scan’s which they shouldn’t be then this will put extra load on the disks.
If everything else looks good it’s time to get the boss to open their checkbook and buy more disks, or faster disks.