SQL Server 2005: Error: SQL Server don´t exist or Access denied

Hi!
I developed an application on VB6. It was funtioning with SQL Server 2000, I
migrated to SQL Server 2005, and when I run some process (a long process) the
following message appear:
"-2147467259(80004005) DBNETLIB Connection Open Connect No existe el
servidor SQL Server o se ha denegado el acceso al mismo"

Sometimes the following messages appear:
"Tiempo de espera agotado"

I don't know What can I do? Because when I used SQL 2000 I run the same
process but I didn't have these problems.
Someone can help me?

Thanks!
Evelyn M