At some point - you'll need to stop IIS logging to the C:\
Open the SharePoint 2010 Management Shell
The IIS snap-in first needs to be loaded using Import-Module WebAdministration
Here is the command to change the log file directory for the default web site:
Set-ItemProperty 'IIS:\Sites\Default Web Site' -name logFile.directory -value 'D:\IISLogs'
Now, open up the IIS Manager console, expand Sites, click on the site you changed the log directory for and double click on Logging under the IIS section. You should see the new directory location under Log File
You will need to perform this on each server in the farm.