The service desk is located on the top (2nd) floor of building 55. This was the best place we could find within the time and other contraints imposed on us. Signs outside and inside the building guide the users to the service desk.

Arguments in favor of building 55:

• It is accessible from outside CERN (can be accessed without an access card)
• It already hosts the registration service and services providing access cards, film badges and car stickers; so it's a mandatory stop for everyone having some contractual relationship with CERN.
• Enough space could be made available to locate the service desk. 

This is what it looks like (click on the plan to get map view):

In building 55 equipment is in place for 6 service desk agents; a backup location in building 73 (5 mins walk) with callcenter connectivity and necessary hardware to host 4 service desk agents is ready in case of loss of network or power in building 55.