How are you achieving high availability in the old infrastructure?

I'm not sure what you mean by 'shared server'

With a SAN cluster nodes can be, and often are, located at different sites. The key thing is a 500ms failover timeout on the heartbeat link. With mirroring there is...

With Mirroring, assuming either SNAC or the Failover Partner setting in the database connection string are used, the application will keep trying the Data Source, then the Failover Partner,...

I think mirroring with a Witness is the solution you require.

Utilising a third server as a witness allows for automatic failover.

All DB connection strings need to have an additional parameter...

The error you mention is quite generic and I have certainly seen it occur when all Network protocols have been enabled.

My guess is that the account this runs under from...

To create the mirror you restored the transaction log with the 'No Recovery ' option.

This means you can not query the mirror as it is not online. You can think...

I have seen these errors before.

Are you running the same Patch level of SQL Server Express Edition as the Principal and Mirror servers?

I was running my Standard SQL servers (Principal...

With the use of a Witness server Mirroring can offer automated failover (it's pretty quick too).

SQL Server native client and .Net 2.0 onwards provide a 'Failover Partner' parameter to support...

DTS support was not ported across to SQL Server 64-bit version.

It is supported as legacy by 32-bit SQL Server 2005.

64-bit SQL server 2005 does not support DTS. You will need...

It's Virtual Centre and other fancy add-ons that cost the big bucks. You can install Virtual Infrastructure Client for nothing...