run database slower

I have database, Visual basic as front end, and sql server as backend, the reports are using crystal report. Recently, the user complain it is too slow to run, it took a long to load the data, anybody help me? Thanks in advance.How can i check what cause this problem. I check the active log file, it didn't help.|||You have to look a couple of things:
1. Design of the database
2. Indexes
3. Execution plan of your queries.

Without this info, it is very hard to provide any help.|||It is sometime very slow. I will check these you posted as this system was design by somebody else. I had question, can this sql server perfome slower in some case that is not relation to design that you said? I want to make sure it is not sql server problem. So i will only check the design. Thanks.