MSP help desk

Hi.  I am a small MSP and so far have not really needed a way for customers to send help requests via an icon in the system tray or something similar.  they just call or email us.   But as things get bigger, I am finding it would be more helpful.  I am looking for a something that would allow the customer to click on an icon in system tray and they could send in a service request.  This program could also be monitoring the desktop for issues.   Also, could be integrated with the anti-virus program.  Looking for ideas and something affordable, being a small company here.