My company sends out HTML email newsletters to registerd subscribers. 
Some are complaining that they sometimes get an unknown error when
trying to open it and some complain of corruption and they cant delete
or move it.
One user said they use Outlook 2000 and Exchange 2000.

Many people and some e-mail services, especially those run by security
conscious administrators at a company will automatically block HTML
content because it has so much potential for carrying malware.

Merely opening an HTML message can infect a computer.

I'm not saying your company does anything wrong, just pointing out
that best practices, including those published by Microsoft often call
for opening e-mails only in plain text.

The various filters to enforce these rules can cause problems because
of security settings.