Me again with an update. Here's what happened:
1. I restarted CS4, opened a file (#1), printed it. No problem.
2. I opened another file (#2 )and tried to print, no printer found.
3. Closed all files, opened the last one again (#2,) still no printer found.
4. Close all files, opened the first one (#1) that I sucessfully printed and printed it, even resized to 60%, no problems.
5. Close all files, open #3 and even though the CS4 print dialogue shows it ready to print, when the XP print window pops up, the z3100 is there, but I can't select it.
6. Close all files, open #1 again and this time no printer found.
7. Close CS4 and restart, open #3, set prefs, print again with no problem.
Not sure if this is qualifies as a pattern....