The problem was solved for me -- sort of. Printer is a HP PSC 2110.
Tried a lot of troubleshooting with HP TSRs, and not much worked. Rebooted a
zillion times, did a clean (scrubber) uninstall, updated firmware, etc.
Finally, they recommended using an alternative driver, deskjet 990C. That
worked -- the 2110 prints successfully. Don't have all the "tweaks" that the
2110 driver has, but it certainly meets my printing requirements.
It also shows that it was not a hardware problem, a USB cable problem, or a
printer port problem. It is a software problem! Clearly a problem with the HP
drivers interfacing with Win XP SP2. The puzzle is that most users do not
have the problem, but some do -- so their has to be at least a 3rd variable
in the equation somewhere causing the problem. Nonetheless, I would hope that
HP updates their drivers to take care of the problem permanentlly.
In any event, the 990C driver probably won't help you with the Deskjet 815C.
If you haven't tried it already, I would suggest installing the Deskjet 550C
driver as an alternate. The 550C driver is considered a universal, workhorse
driver. (After install, be sure to go into the ports settings -- right click
properties on the 550C icon -- and select USB if you are connected that way
-- it defaults to an LP1 port.)
I hope this helps.