Fax problem with HP Officejet T series

I’ve been having a lot of trouble with my printer/fax and it may be caused by
something MS XP is doing.

For years, the HP all-in-one T-45 has performed normally, now it will not
fax from a word document in the ‘puter (faxing from the unit itself operates
normally.) Printing from same word document works fine. We have
uninstalled, manually, and reinstalled MANY times with the same results: The
fax function works fine for a while, and then will not work. Necessary fax
files have mysteriously disappeared. Restoring to the date and time when all
was working does not cure the problem, still no fax capability.

I have an HP scanner 4200C installed and HP says that should be not problem.
Gear: Dell Dimension 3000, Win XP VER 5.1, Service Pack 2.

Any and all help greatly appreciated.