After installing SQL server 2005, the .net application are not able to connect to the sql server, or not able to remote connect to it.
On your server side
1. Enable remote named pipe or tcp
All programs | Microsoft SQL Server 2005|
Configuration Tools | SQL Server Surface Area Configuration |
Configuration for Services and Connections |
Remote Connections, choose either enable TCP or Named Pipe or both.
2. Start the sql server Browser
go to services.msc, find service "SQL Server Browser", enable it and restarted, also, go to SQL Server Configuration Manager, check properties for SQL Browser service, in Advanced tab, make sure it is active.

That generally should take care of the problem.
why ?
Microsoft, nowadays assumes that it is safer to switch off all the features of its applications and operating systems by default.  If anybody (adv users) needs its they will have to switch it on, and assume all the responsibilty