Sql Server 2008 R2 express - Instance available to all users?

I am attempting to build a sql server 2008 R2 installer for XP (yes i know but the place i work at still uses XP!)

Here is the problem:

The Sql Server Express 2008 R2 installer works fine for the Admin user of the PC who installed it. The software that uses the Sql Server runs brilliantly.

osql -L returns the correctly named instance.

However when another user logs into that pc, the Sql Server is available - but the created Instance "MyCoolSqlInstance" is not.

osql -L returns an empty list.

How do I make the instance available to all users of the PC? (there will be Admin and non Admin users). 

cheers

Buzz