- You said that drive C: is 68 GB, but how much disk space was free before running the stored procedure?

- Have you used Sysinternals FileMon or Process Monitor to log disk i/o events and determine which file(s) are growing?

- Does the stored procedure contain just select statement, or it is also inserting or updating as well?

- Is the stored procedure selecting into a temporary table or table variable?

When installing SQL Server, by default, the master and tempdb databases are located on the same drive as the application.

For example:

C:\Program Files\Microsoft SQL Server\MSSQL.1\MSSQL\Data

Also read this article: Troubleshooting Insufficient Disk Space in tempdb

http://msdn.microsoft.com/en-us/library/ms176029.aspx