About sql table(Allow nulls)

If you do not allow nulls in a column, then SQL has to do one of two things when you insert a new record, but don't supply a value:
1) Complain and reject the insert with an exception.
2) Use a default value for the column, if set.

Numeric values have a default of zero (unless you change that in your table definition), but others cannot be assigned a default value because there is no sensible default that can be applied.

In your windows application, if you do not have a value, then I would suggest that you supply one which makes sense to your windows application as a "no value assigned": for a string column, that would be an empty string, for numbers I generally use -1 or the maximum value, or I allow nulls and check for them!

Check your table definition, and see what default you have allowed.

BTW: You can post table definitions here very easily: just go to SSMS, right click the table, and select "Script Table as...", "CREATE To...", "Clipboard". You can then paste that directly into your question:

USE [Testing]
GO

/****** Object:  Table [dbo].[MyTable]    Script Date: 09/01/2013 09:08:13 ******/
SET ANSI_NULLS ON
GO

SET QUOTED_IDENTIFIER ON
GO

SET ANSI_PADDING ON
GO

CREATE TABLE [dbo].[MyTable](
	[Id] [int] IDENTITY(1,1) NOT NULL,
	[Name] [varchar](50) NULL,
	[Model] [varchar](50) NULL,
 CONSTRAINT [PK_MyTable_1] PRIMARY KEY CLUSTERED 
(
	[Id] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
) ON [PRIMARY]

GO

SET ANSI_PADDING OFF
GO