What do

Don't panic. Whatever you do, don't panic. But let me congratulate you for keeping your computer running on excellent condition where you have 4GB of RAM and the system is using only 0.2GB of the available resources. About the red line on your disk D. Let me just say that Windows 7 has an interactive way of communicating with its end users like us and once any of the partitioned disk reaches below 15GB of usable space it will indicate to us with a red line telling you that you have less than 15GB space in your HDD to store your data like photos/songs/movies/videos and etc. In this case, your Disk D is having 12.4GB of space left for storage. So, you may want to go in there and see what you have stored and shift them to another location of your HDD which I am sure has other partitioned locations like E:/F: (Note: Please do not store in C:). I would recommend E: (Disk E) so just create a folder or two there and start moving your load of date in D to E. I believe you have downloaded big files like movies or video clips and if I may add some are downloaded unknowingly. Sometimes some sites just stream and download at the same time. Other than that, I believe you may have quite a number of software programs inside it. But whatever it is, don't panic just reexamine the contents and decide what you want to do with it. Either move or delete it is your choice. On Delete FYI when you delete any files at any locations, please remember to follow up by clearing your "Disposal Bin" otherwise your disk space will reflect the same size as before where you have not "deleted" your files yet. So, do clear it and only then will you see a reduction in your disk space. I hope I have been clear here and should you have any need for clarification or explanation, please feel free to comment back.

Are you talking about your hard drive being full? What size is it? Exactly what did you do with the recovery disks?

Download Recuva install it and scan the drive.