Contact form is acting like an application

I have created a contact form, but for some reason when I click the submit button I get a popup window that asks if I would like to download the php file.  Why would this be happening?  My code is the following.  I had this happen to me before, but I can't remember what the solution was.

<table width="100%" cellpadding="5" cellspacing="0" class="contactus" bgcolor="#effde1">
<tr><form action="/dev/scripts/contactus.php" method="post">
<td>
First Name:
</td><td>
<input type="text" size="20" name="strFirstname">
</td></tr>
<tr><td>
Last Name:
</td><td>
<input type="text" size="20" name="strLastname">
</td></tr>
<tr><td>
Company:
</td><td>
<input type="text" size="30" name="strCompany">
</td></tr>
<tr><td>
Email:
</td><td>
<input type="text" size="20" name="strEmail">
</td></tr>
<tr><td>
Phone:
</td><td>
<input type="text" size="14" name="strPhone">
</td></tr>
<tr><td valign="top">
Comments:
</td><td>
<textarea rows="5" cols="35" name="strComments"></textarea>
</td></tr>
<tr><td align="center" colspan="2">
<input type="submit" value="Submit">
</td></form></tr>
</table>