Error code 5100

Here's what worked for me in solving this error code 5100 problem. Not only was I unable to print anything because I kept getting that code, but there was a thunking sound when the carriage stopped moving, and every time I turned the machine on it was feeding through one sheet of blank paper. I was about to go out and buy a new printer of a different brand because the Canon was unusable.
Then, fooling around with the machine's menu and settings buttons, I came across an option, I think it was in the Maintenance menu, to Restore Factory Settings. I chose that option and it asked me again if I wanted to restore the original factory settings, Yes or No. I said Yes, and immediately the printer started behaving itself. No more 5100, no more funny noises, no more blanks sheets of paper. It is back to its old self again.
That still doesn't solve the problem of spending a fortune to buy ink cartridges, but I just hooked up my old Panasonic 24-pin dot matrix printer to the printer port of my CPU (a ribbon lasts me a year and costs less than $15), and the Canon to another port, so now I can print the day-to-day stuff on the cheap dot matrix and use the Canon for the important stuff.
I don't know if restoring the factory settings will work for you, but it was the solution in my case. Hope it helps you out.