Assuming your are running Windows 7 and the Disk that is showing red is the system drive C:, try an initial disk cleanup process which will check for redundant user files / system files which you can select to delete
Try:
- Select 'Windows Start'>'Computer'
- Under the Left-hand column in 'Computer' window RIGHT-Click 'Local Disk C:' and select 'Properties'
- In the 'Local Disk C: Properties' window click 'Disk Cleanup'
- A list of potential files for deletion will be searched for and displayed. Tick the ones that you want to delete such as 'Temporary files', 'Recycle bin' and maybe 'Download files'
- You can also click 'Clean-up System files' to remove redundant system files
- Click 'Ok' to complete
Suggest you then use the same 'Computer' browser search / check out the User files directory located in the Login users accounts area at C:\Users\'Login user name'. If any files that you deem them ok to delete, then delete and they will be moved to the Windows 'Recycle Bin', which you should then browse to (usually located on the Users Desktop) and right-click on and select 'Empty Recycle bin'
Hope that helps.