Two additional questions…
The admin page seems te be accessible from the other computers so I thought it was working fine. But still it isn’t appearing in iTunes on all the computers in my network.
When you say “all”, do you mean all or any? If “all” is more accurate, then how does the configuration on the machines which can access the shared library differ from those which cannot?
Also…and this is a rudimentary check, but it has proven to be the culprit in the past…have you enabled the bit within the iTunes client which searches the local subnet for shared libraries?
If you have not already done so, you may want to read through this earlier thread regarding a similar issue. The solution never really revealed the true culprit — other than being a Windows idiosyncrasy — but there are several useful troubleshooting tips which may aid in diagnosing and eliminating possible causes.