Can I encrypt a field of a table using stored procedure?

Can I encrypt a field of a table using stored procedure in SQL Server 2005 Express Edition? And also, how to decrypt it through stored procedure?

Using the encryption functions.

In Books Online, look up the topic: Encryption, sub-topic: Functions.

You can download a version of Books Online for SQLServer Express from:

SQL Server 2005 Express Books Online Express Edition
http://msdn2.microsoft.com/en-us/library/ms165706.aspx