SQL Server and its files are taking up a large amount of disk space.
SQL Server automatically creates a database called tempdb for use as a workspace.
The simplest thing you can do to shrink tempdb to its minimum size is to shut down the SQL Server service and then restart it - there is no need to reboot.
If you need to control where the SQL Server files are located, e.g. not the C: drive, then you should consider installing a fully licensed copy of SQL Server (SQL Server 2005 Express Edition is free and is certified to work with WebWatchBot).