Sorry to say this, but the answers you get are = to the information you give. No one wants to touch this thread because you have a strange problem and very little information (Windows XP? Vista? Do you have other hard drives? Is Windows installed on that hard drive? What do the other drive icons look like? About when did the problem start? Have you had any other problems? etc etc etc...). The key is providing as much information as possible.
Just so you know, the red x doesn't mean anything because Windows doesn't use a red x icon for your hard drives. I think this is one of the reasons no one has helped -- it doesn't make sense. I'm going to take a stab at it though...
Since Windows does not use a 'red x' icon for your hard drive under any circumstances, what has probably happened is your drive icon has been changed. Why? Well, I think you have a virus. Who knows though, maybe it was a prank... maybe this is some coincidental registry or shell icon corruption... But I think you should
scan your system for viruses.
Also, I know you don't know much about computers, but formatting your drive would have been a really 'stupid' thing to do. It deletes EVERYTHING including your files, programs and even Windows itself from your hard drive rendering it unbootable. Lucky for you though, Windows can't delete itself so it won't allow you to format the drive it is installed on... So in the future, make some effort to look online before you use something that you have no clue about. You
must be more cautious!
As for a fix, a virus scan is a must. But the settings for the icons are located in Start > Run and type regedit.exe then press the enter key. Navigate to HKEY_LOCAL_MACHINE / Software / Microsoft / Windows/ Explorer / DriveIcons
There is probably a custom entry of "C", which will appear as a folder to you in regedit. Click on the folder and it will reveal the icon details. Chances are there will be one or more values over in the right window pane. These values tell Windows to replace the default icon with a custom one. This entry is was probably added by a virus/malware program to annoy you.