Its most likely a permissions issue. Is your page running as the anonymous user (IUSR_MachineName)? If so then you'll have no luck because that's a local account not a network account, so it won't have rights to any files on the network. Stick with anon, but either:
1. change the anon user to a network account that has permissions to that network share and re-try.
2. create a local account on the network PC with the same name and password as your anon user and give it rights to log-on locally to that PC (often us lowly developers don't have rights to do that sort of thing :)
If your page isn't running as the anon user then you'll not have any luck either because of NT's lack of delegation - switch back to anon and follow the advice above and you should be OK.