Well, to clear it of data (if you are _sure_ that you don't need that data) you could either
1) Erase the files, or
2) Format the partition, using Disk Manager (right-click on the partition and format it) or just right-clicking on the drive in Windows Explorer.
3) If the data is confidential and you want to make sure that it is unreadable, do a Secure Erase if it's an SATA drive or use an overwrite utility if it's an IDE drive.
As to making it a slave driver, give it some reports and a deadline. wink.
By slave drive, do you just mean one that holds data but is not your OS drive? The simplest way to do that is to install the OS on another drive. Poof - this drive is not your OS drive. The only time that "slave drive" applies is if this is an older IDE drive. Can you either let us know if it's SATA or IDE, or give us the drive model number, or post a photo of the connectors, or describe the cables? That last list was in decreasing order of usefulness.
Formatting it will erase all the information on it (okay, it's recoverable from a quick format, but the idea is don't do it if you want the data).
The most reliable way that I have found is to go to the Disk Manager (Control Panel -> Administrative Tools -> Computer Management, tab Disk Manager), make 101% sure you identify the correct drive in the pictures at the bottom, right-click on the drive, delete any partitions if there are any, create a single partition with the full size of the drive, and format it as NTFS.