IE7 and the recently-released IE8 are the only versions of Internet Explorer that are supported by Windows Vista. IE6 was not designed or updated for Windows Vista, is incompatible with the operating system, and checks the operating system during installation, refusing to install on any operating system not explicitly supported. The HP representative should have been aware of this.

You could try a System Restore back to the last time it worked properly, before HP's representative made whatever changes he/she did, and then re-address the printer issue. If that fails, you could also try running System File Checker to see if it is able to repair any corrupted system files that might exist.

Hope this helps,
John