The crude way of doing this on the old slave drive is simply to delete the files using windows explorer.
You need to be careful though because some programs may have kept your data in places other than your My Documents. Camera Photo software for example notoriously retains pictures in their own folders. For example c:\program files\sony\pictures. If you delete the folder program files on the old drive the pictures disappear as well.
You need to make sure that all your old data is stored somewhere safe. For example have you transfered your e-mails under the new sysytem?
The best way to get the most out of your spare drive is to move all your data temporarily to folders on your new "c" drive. Once you are sure that it is all safe, simply format the old drive, create your data folders on it and transfer your files back.
Before you do the format I suggest you do a search on the old drive for all files and folders putting *jpg, *.gif, *.bmp (Pictures) then *.doc, *.xls, *.ppt (Office Files) and any other file extensions for other progams you use. This should make sure you pick up any data that has been hidden away on the old drive.