In order to verfiy the current memory restriction on your SQL Server / SQL Express you can run the query "currentmemory.sql" attached.
To define a new memory restriction you can run the query "setmaxmemory.sql".
The query is currently set to 256MB, you can edit the value depending on your needs:
EXEC sp_configure 'max server memory (MB)', 256;
-- here the max memory used by the sql instance is 256MB
Further details can be found in Microsoft KB 178067