Question no ones asked yet, what version of Windows O/S you running?
As far as the pop up your getting about a program offering to clean it up. This suggests to me that you have some malware on your pc. Do as hc4361 suggests and download and install Malwarebytes, update its definitions and run a full scan. You might need to run it with your pc in safe mode.
By all means download and install CCleaner. It's good at deleting unnecessary dross, but do not use the registry cleaner section. Windows copes quite well with a scruffy registry and I see no reason for you to edit the Registry.
VHG has the best answer for the icons on your desktop. I tend to put all my shortcuts into a folder on the desktop. Keeps it tidy but easily available.
De-fragging your drive has become almost redundant now. Windows 7 and above will keep themselves tidy now. Even if you did do a de-frag, as soon as you used the pc it will become defragmented again.
If your having difficulty reading the screen, you do realise you can alter the sizes of the fonts you use. You can alter your preferences in Control Panel/Display.