Another program could be interfering with Messenger.
Do you have anti-virus or a firewall? Try disabling them.
Also, this bit is a bit more advanced, but you could try starting your computer up in
Safe Mode. This runs your computer with the absolute minimum amount of programs.. so you can open up Messenger and see if you have any problems with it. If you don't, you know that something is interfering with it on your normal computer mode.
Note: When choosing an option for Safe Mode, choose 'with Networking', which means you'll be able to connect to the internet. Follow the instructions carefully and if in doubt, restart your computer to return to normal.
Degragging your computer isn't likely to do much. You could try
CCleaner which clears temporary files off your computer, to speed it up, including Messenger-related files (just make sure Messenger isn't running).
Good luck.
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