Show us your code.
Because here is a demonstration to the contrary:
*** FILE: demo.html ****
Code:
<html>
<body>
This is the main page.
<br/><br/>
<iframe src="iframe.html" style="width: 100%; height: 300px;"></iframe>
<br/><br/>
More main page
</body>
</html>
*** FILE: iframe.html ****
Code:
<html>
<body>
This is iframe.html.
<form onsubmit="alert('form submitted'); return false;">
<input type="submit" value="Click here to submit the form"/>
</form>
</body>
</html>
It works just fine in MSIE.