Very weird problem upgrading an old app

[dgk]

We have a VS2003 app that we've had to make some changes for, and then
we copied the resulting dll to the production server. There is a login
screen that appears with no problem when the app is started. On some
of our users machines, clicking the image that triggers the postback
for the login works just fine. On others, nothing seems to happen.
Javascript does still work in that case because the validation
controls still work.

All the machines are XP, and the server is running 2000 Advanced
Server. We have a test machine, also running 2000 and all the machines
work fine hitting the new version on the test server. In fact, we
tried Firefox on one machine that did not work with the production
app, and it works as far as logging in. There are other issues because
the app was written for IE, but logging in works. IE is version 7.

If we reinstall the old version of the app, it works fine from all
machines. So something we did affected it, but we have no idea what.
No changes were made to the login page.

None of us were here when the app was written. It uses some Microsoft
codeblocks and Crystal Reports. We've pretty well gotten rid of the
Crystal Reports and are using SQL Reporting Services.

Right now we've fallen back to the old version but we're going to try
again with the new one tomorrow morning, if just to gather more data
on what is happening. That server is indeed very weird, as is the app
which lives in the IIS root directory instead of another virtual
directory. We tried moving it once and that didn't work.

Any ideas greatly appreciated.