What's the better desktop database to work with .net ?

What do you mean when you say 'desktop databases'?

Are you looking for databases like sqlite, Microsoft Access, MS SQL Compact Edition, MySQL, etc...?

Yes, databases easy to install remotely, mainly assembled over data files instead o more heavy and sophisticated engines.
Actually my system must to be downloaded and installed by people with almost no tecnical knowledge, so the database must be installed at the same instalation package the exe remains in.

Did you consider using xml? It would be simple to install.

Drawbacks:
 - you need to know xml well
 - sql queries are not working here; however, LINQ can be used
 - not really suitable for big volumes of data

See:
http://www.codeproject.com/KB/linq/XmlbasedPhonebook.aspx
http://www.techrepublic.com/article/a-simple-xml-database-for-your-windows-applications/5116182
http://forums.asp.net/t/910853.aspx/1?c+using+XML+file+as+database

It means to persist the data in XML files ?

The drawbacks make it very difficult  to use it in my aplication, it's going to be migrated from an existing 32 bits one...

I've planned to use a small database that could be rapidly installed (no special efforts in install it) like Acess.

But I don't know what is the better option.

Hi,
consider any of the following Express Edition of, Microsoft SQL Server 2005/ 2008/2008 R2 edition

revert me back if you have further any query !

Hello

Consider this scenario
Someone downloads my system instalator, simply install it and try to start using it... the person has no tecnical knowledge

The instalation package could install MSSQLServer at the PC - automatically  and continually working for him with no interferences ?
I guess no....

For you to better know what I mean - maybe try it by yourself at
www.espiriplug.com.br 
It runs with VFP technology with .DBF - since the user install it no issues about databases...

I intend to have this ease with .net platform and using web forms.

i understood your requirement. you can consider using any of the  Express Edition of, Microsoft SQL Server 2005/ 2008/2008 R2 edition.

please follow the instruction in the link

http://msdn.microsoft.com/en-us/library/31kztyey.aspx

if still you have problem you consider checking the link below
http://www.manjuke.com/2010/05/deploying-windows-applications-using.html

NB:  Publish a ClickOnce Application using the Publish Wizard  in visual studio 2010/2008/2005

Hello

I've saw the contents very quickly
But it's possible to install SQLServer Express edition (configured)  from inside my own instalation package - is it right ?

Certainly yes. You need to follow the steps.

Please read full article you will get the solution. Just relax  and follow what is mentioned in the article.

http://msdn.microsoft.com/en-us/library/31kztyey.aspx

http://www.manjuke.com/2010/05/deploying-windows-applications-using.html

Just one last thing

The users  not even know MSSql Server Express is running at their machines, ok ?

(optionally the instalation package - if developed for web -  could be installed at web also ?)