Citrix receiver has an annoying habit of prompting for user credentials when it try's to connect to a storefront store. This can happen when a user is not even attempting to use the Reciever.
This behavior is highly annoying. Especially if there is no requirement for the user to connect to a store.
The Prompt can be disabled with the following registry key
32 Bit - HKLM\Software\Policies\Citrix
64 Bit - HKLM\Software\Wow6432Node\Policies\Citrix
Add the following DWORD entry and set to 0
EnableX1FTU (for older Receivers the entry is EnableFTU)