Printers Off Line

In my 4-PC-network the Vista-64-based PC is attached to a HP Deskjet-printer.
The other 3-PC's (all XP) are configured to work with this printer through
the Vista-PC. This worked excellent with HP-software installed, up untill
this week, where all of a sudden all the three XP-PC's showed the printer off
line and unable to print documents. Nothing changed in the network. The
strangest thing is that after re-installing the HP-software on one of the
XP-PC's the printer is renewed accessible, but a day later the off-line
situation is restored.
The printer operates o.k., when used with the Vista PC. Share-paramaters are
still in place and facilitate printer-sharing.

Could it be - just guessing - that a recent Security Update from Microsoft -
blocks this printer-sharing mode? Any ideas?