After re-installing SQL server, app did not work

For some reasons, I uninstalled and reinstalled SQL server in local pc. Now, none of my app is working. My pc is XP professional with sp2 and VS.NET is 2003.
I spent a couple of days but no luck. It drived me crazy!
Help!!!did you add the MACHINENAME/ASPNET user account to the db?|||In Web.config file, I use "sa" in connection string, do I need to add ASP.NET account?|||if you are using the useraccount "sa" to connect to the sql server then the user "sa" should have access to the database.