Icons Changed - Turned To Square (undefined)

Shutting down a game many of my desktop icons e.g. recycle bin, network environment and all the shortcut folder icons created by myself suddenly turned to square (undefined) default icons that are familiar to us from Windows 95. When I click 'My Computer' I discover that also the icon of drive C and the floppydisk 3,5 drive icon have changed to default icons. Additionally, all the standart yellow foldericons now show default icons.

Before I have occasionaly used register cleaning programmes such as the free CCleaner and the Uniblues shareware Registry Booster with no problems. I have always taken a complete backup of the Register in case something goes wrong. Also, I have understood that no change in the register takes effect unless you restart the computer. Am I wrong here?

After a quick check I notice that all the installed software function correctly. All written documents are there and they open correctly in their respective programmes. I've tried to change the icons by right clicking "properties/customise/change icon" but it doesn't work. The computer does not allow you to change the icons that vanished. When you try to change the icon, you are by default navigated to system 32 folder with shell32.dll file. I presume this is the main icon library of xp. All the pictures are present there but those that are "broken" approx. 12 of them showed at the beginning of the list cannot be used anymore, for instance the standard yellow folder icon among them. The rest of the icons (say 100 or more of them) are available..............