When I upload my forum to a free host in aspspider.net:
http://aspspider.biz/tuankiet/forum

I have a problem in connect to the database. I can't connect to the database.

The connection string in web.config file is:
<connstr>user id=xxx;pwd=xxx;initial catalog=yaf;timeout=90</connstr>

The connection string the host give me is:
Data Source=.\SQLExpress;Persist Security Info=True;Integrated Security=SSPI;Initial Catalog=username_datafilename

I tried both but I can't connect to the database

How can I do it?

Have you asked your provider for connection string examples? Some providers have unusual Conn strings. Just a thought.

Bump!
Installing yaf on www.aspspider.com is piece of cake. All you need is bin package and a empty ms sql 2004 express detached mdf.
The service is absolutely free but usefull for evaluation purposes only, since all files get deleted after 60 days upon registration - no warnings, no backup, no notification.

Thumbs up!