HP All-in-one; error while printing, only half a page printed.

[the big e]

On startup, the HP program gives me a message saying it can't detect the system tray, and then force-exits. (though for some reason it doesn't do that every time I boot)

When I try to print something, I get the same problem as before, with a crash in the middle of the job. (half a page if it's a color print of a web page, two-plus pages if it's a wordprocessing file, and then it hangs forever, perhaps printing out a last line or two after I cancel the job)

Also an erroneous message that the cartridge is empty (I replaced it very recently)

Works perfectly in Windows, though.

[the big e]

So I still have the same problem, even after attempting reinstall of HPLIP. Is there a way to roll back my printer driver to the previous state, in case what happened is some system file got mangled when I tried to instal something that wasn't ubuntu-specific?

What it still always does is give me a message that says "no system tray detected. exiting" and also occasionally a message that says "a system problem occurred" which is probably related. How do I roll back to the appropriate earlier state?