Requirements:

• tiny WinForms client app (C# 4.0, WinForms or WPF)
• a few users working simultinausly
• no database service at all - the whole engine as *.DLLs inside client apps
• database available as shared folder on one computer
• at least simple concurrrency checks
• compatible with nHibernate or EntityFramework / NET 4.0
• backup as simple as copying files from shared folder - assuming no running clients at the moment
• no stored procedures/triggers required
• data size - a few tables and a few thousands rows after 2 years

Nice to have:

• user access rights
• encrypted data

I'm trying to choose between:

• MS Access
• SqlLite
• SqlServer Compact Edition.

Can you recommend which one should be the best for these requirements?