This question is asked a lot and the answers will vary based on who you talk to. Access databases containing thousands of records that begin to see heavy traffic daily will begin to bog down. Microsoft Access is an inexpensive database solution for small projects. Access is not a database for Internet. A handful of users will crash Access. If you are serious, use SQL Server 7, Oracle, or some other database solution made to "server" data.
However, your Access database will certainly survive a lot longer if you incorporate techniques of normalization. If you are unfamiliar with normalization, please read the following
If you have incorporated all the techniques of normalization and your queries are extremely slow and using heavy server resources, you need to invest in a more efficient database. The next step up is generally Microsofts SQL Server Database.