I don't have an Arox printer or Argobar, but I've seen similar messages from other programs.
First, I'd open a file in Argobar and make sure that another printer has not been selected. If the printer name has changed since the file was saved, the program probably selected your default printer, with far different attributes (higher resolution, different fonts, no thermal options).
Also check the printers properties (probably under an "advanced" option) to make sure the driver is installed for the correct model. If your printer supports label sensing other than "Media with Gap", the driver should show other settings. If a label is fed consistantly to the same spot when you Form Feed, then your sensor is working. It might take two From Feeds - one to advance the leading eadge to the printhead and one to advance the trailing edge to the tear bar.
If you just installed XP SP3, you might need to re-install the driver. You can download from Argox, but I prefer Seagull drivers, at http://www.seagullscientific.com/aspx/thermal_printer_list.aspx#Argox
. There are several +2000 models listed. These work with XP SP3. You should be able to set many attributes in the Proerties and Preferences.
Occasionally, one or two files may be saved incorrectly and become corrupted (but never ALL files). In these cases, it easier to start a new file, with the proper label size, and copy & paste (if your software lets you) from the old to the new. Once I get files working well, I always copy them to a another folder.
By the way, if you have already printed with Direct Thermal settings, but had a ribbon loaded, the print head has not been damaged. Direct Thermal should turn off the warning when the ribbon sensor doesn't find a ribbon, and probably changes some of the heat attributes to optimize for ribbonless printing, so the quality with a ribbon is decreased.
The printhead can be gently cleaned occasionally with 70% Isopropal Alchohol on a Qtip, but never touch the printhead with a hard object. One tap with a metal object while the printer is on would probably destroy that part of the printhead.
I hope this helps!