We have Spiceworks setup. People email an address we have setup for help. Spiceworks sucks in the email and creates the ticket. Other than when we respond out of it people don't know anything about the ticketing system. You can really change how and when Spiceworks sends out emails, making it almost invisible to the end user. I recommend some type of ticketing system just for tracking purposes.
@JaredBusch that Windows license thing is a huge deal. Everyone always blows it off with a "SMBs only run Windows" but that's so not true, especially in this day and age with virtualization. And any technical MSP knows to use the right tool for the right job. Sure 99% of SMBs run Windows but 100% of SMBs don't want to waste money for no reason. Running Windows doesn't mean you have spare, otherwise useless licenses sitting around. The cost of SW is actually quite high when you consider the licensing headaches. And since you either need a VDI license, a server license or to run it on a physical desktop it is far more costly than people really admit.
Running SW "right" means a Server 2012 R2 license virtualized with some serious disk subsystem to keep it running smoothly. That's far from trivial if you are an SMB. That's more cost than many put into their entire IT infrastructure!