Microsoft Access 2003 vs SQL Server 2005 Express
Few things to consider with using SQL Express vs Access 2003:

SQL Server is limited to 1 CPU
SQL Server is limited to 1 Gig RAM
SQL Server is limited 4 Gig database file (2 Gig on Access)
Creating front end logic for users will take a longer time than Access 2003
Using SQL more reliable as you can do point in time recovery

More comparisons among the two in this link from ZDNET (UK)