I am using Windows XP and all of a sudden, when I clicked on the shortcut to MSN Messenger one day (after starting the computer), it didn’t respond. It seems to be "working somehow" (light indicator showing something is happening and the computer is buzzing) but the Messenger window never shows up. I have uninstalled it several times, rebooted, installed again, but to no avail. I have also removed the shortcuts and created new ones. Nothing seems to work. The process is running though – msnmsgr.exe. I don’t know why the computer started behaving this way. I have an antivirus, don’t go to any suspicious sites, etc. Also, quite recently, my Firefox stopped remembering and suggesting form entried used in the past although it is set to do so. Is it possible that my Windows XP need reinstalling or so? I have had the computer for 2 years. I have a RegistryCleaner that I use from time to time (and have used it now again with no improvement) that always finds lots of errors, etc. and fixes them. (Although I suspect it is quite possible that the registry cleaner may have caused some mess too.) If you have any suggestions, I would be grateful. Thanks a lot in advance.
Adding some details:

Thanks, I have done so and all went fine. Except for the fact that it has not solved my problem, unfortunately. I have tried installing it, uninstalling it, cleaning the registry, etc. and all in vain. I have really no idea why it is doing what it is doing. All of a sudden. Any other idea? Thanks a lot in advance.