override sidebar() function in your theme, change HTML and CSS to fit it in sidebar(I just copy/pasted the html, you can remove table tags):
function sidebar()
{
$sidebar=@$this->content['sidebar'];
if (!empty($sidebar)) {
$this->output('<aside CLASS="qa-sidebar">');
$this->output_raw($sidebar);
$this->output('</aside>', '');
}
$useid=qa_get_logged_in_userid();
if (!isset($useid))
echo '<h1>
Log in
</h1>
<form method="POST" action="./index.php?qa=login&to=index.php">
<table class="qa-form-tall-table">
<tbody><tr>
<td class="qa-form-tall-label">
Email or Username:
</td>
</tr>
<tr>
<td class="qa-form-tall-data">
<input name="emailhandle" id="emailhandle" value="" class="qa-form-tall-text" type="text">
</td>
</tr>
<tr>
<td class="qa-form-tall-label">
Password:
</td>
</tr>
<tr>
<td class="qa-form-tall-data">
<input name="password" id="password" value="" class="qa-form-tall-text" type="password">
<div class="qa-form-tall-note"><a href="./index.php?qa=forgot">I forgot my password</a></div>
</td>
</tr>
<tr>
<td class="qa-form-tall-label">
<label>
<input name="remember" value="1" class="qa-form-tall-checkbox" type="checkbox">
Remember me on this computer
</label>
</td>
</tr>
<tr>
<td colspan="1" class="qa-form-tall-buttons">
<input value="Log In" title="" class="qa-form-tall-button qa-form-tall-button-login" onmouseover="this.className="qa-form-tall-hover qa-form-tall-hover-login";" onmouseout="this.className="qa-form-tall-button qa-form-tall-button-login";" type="submit">
</td>
</tr>
</tbody></table>
<input name="dologin" value="1" type="hidden">
</form>
';
}