Simple enough. You have three options. Each will work, but you'll probably find one of them fits your needs better than the others.
1. Remove the <?ASPNET_FORM>...</?ASPNET_FORM> tags from your templates (especially a master template) and re-insert them only when/if you need them. Keep the tags as close to the location you need them as possible.
2. As Paul said, create a new template for this page that does NOT include the ASPNET_FORM tags and does not have a master template that includes it.
3. (this is a bit of a hack) Because you can't have a form in a form... close the ASPNET_FORM tag, insert your form, and then open the ASPNET_FORM tag again. The code above would look like:
Code:
...
</?ASPNET_FORM>
<form action="https://www.salesforce.com/servlet/servlet.WebToLead?encoding=UTF-8" method="POST">
<input type=hidden name="oid" value="id number here...">
<input type=hidden name="retURL" value="http://www.website.com...">
<label for="first_name">First Name</label><input id="first_name" maxlength="40" name="first_name" size="20" type="text" /><br>
<label for="last_name">Last Name</label><input id="last_name" maxlength="80" name="last_name" size="20" type="text" /><br>
<label for="title">Title</label><input id="title" maxlength="40" name="title" size="20" type="text" /><br>
<label for="company">Company</label><input id="company" maxlength="40" name="company" size="20" type="text" /><br>
[etc...]
</form>
<?ASPNET_FORM>
...
As I said, #3 is a bit of a hack but it usually works in a pinch.
cheers,
doug.
MVP 2007-2009 - Official Umbraco Trainer for North America -
Percipient Studios