Remote Aspnet Database Access Help

I have my normal SQL 2005 database....and another database (on the same machine) that is the default aspnet database. I use that for authentication of course.
 Everything is working well except now I need to (for a SP I'm creating) access the UserID field in the Users table (without the customer logged in). I know this isn't the best way to do it but....


Can someone tell me how to programatically (from an existing SP) access the aspnet database? So I can just check something against the UserID (providerkey) I already have? The customer isn't logged in at this point. I have their UserID from somewhere else.
 Here is what I am trying to use now. IT Works in Query Manager but throws an error (...in this security context) when I run it in ASP.NET...
SELECT @UserName=(select UserName from  [aspnetdb].dbo.aspnet_Users where UserID=@UserID) 

 
Thanks.