Theres lots around for download. Tomm has a proffesional one - www.usersystem.net/forum register then post in the forum "Free License Requests" and if your accepted you'll get his.
If you dont get that you could always use the not-so-good Ourhabbo one, im not sure where you can download it from anywhere though.
The login page coms weith the usersystem as what it does is:
You type your username and password and press logim. Then if you have created a PHP file to verify who they are etc. and make sure the password is right, you can make it to re-direct them to there members area etc.
But when you download a usersystem it all comes with that.
If you just want a simple form, use this:
<form id="form1" name="form1" method="post" action="">
<label>
<span class="style1">Username: </span>
<input type="text" name="user" id="user" />
</label>
<br />
<span class="style1">Password:</span>
<label>
<input type="text" name="pass" id="pass" />
</label>
<p>
<label>
<input type="submit" name="submit" id="submit" value="Login" />
</label>
<br />
</p>
</form>
Thats just the two forms and a login button, it wont have anything like members area etc.