paste below code in header part :-
<?php $myStatus = Mage::getSingleton('customer/session')->isLoggedIn() ?>
<?php if(!$myStatus): ?>
<style type="text/css">
.blankdiv{background-color:#000;
position:fixed;
z-index: 9001;
top:0px; height:100%;
left:0px;
width:100%; opacity: 0.65;
filter:alpha(opacity=65);}
#popupform1{height: 100%;
left: 0;
padding: 15px;
position: fixed;
top: 0;
width:97%;
z-index: 10001;
}
#popupform1 .applyform{position:relative; overflow:auto;
background-color:#fff;
width:500px;
height:500px; margin:5% auto auto auto;
z-index: 9002; padding:10px; border:10px solid #7F3814; }
#pclose{background-image: url("http://www.americanpowergear.com/close.png");
background-position: left top;
background-repeat: no-repeat;
cursor: pointer;
height: 25px;
margin: 5% auto -6%;
position: relative;
right: -348px;
text-indent: -9999px;
top: 0;
width: 25px;
z-index: 9999;}
</style>
<div id="popupform1" style="display:block" width="500" height="500">
<div class="blankdiv" style="opacity:1!important; background-color: #FFFFFF;"></div>
<div class="applyform"><h1 align="center"> Welcome Login Form </h1>
<p id="contactArea">
<form id="login-form" method="post" action="baseurl/customer/account/loginPost/">
<div class="content">
<!-- <form id="login-form" method="post" action="http://www.americanpowergear.com/index.php/customer/account/loginPost/">-->
<input type="hidden" name="form_key" value="<?php echo Mage::getSingleton('core/session')->getFormKey(); ?>" />
<div class="clearfix">
<div class="registered-users">
<div class="content">This is a demo store. Any orders placed through this store will not be honored or fulfilled.
<div class="border"></div>
<ul class="form-list">
<li>
<label class="required" for="email"><em>*</em>Email Address</label>
<div class="input-box">
<input type="text" title="Email Address" class="input-text required-entry validate-email" id="email" value="" name="login[username]">
</div>
</li>
<li>
<label class="required" for="pass"><em>*</em>Password</label>
<div class="input-box">
<input type="password" title="Password" id="pass" class="input-text required-entry validate-password" name="login[password]">
</div>
</li>
</ul>
<div class="buttons-set">
<button id="send2" name="send" title="Login" class="button" type="submit"><span><span>Login</span></span></button>
</div>
</div>
</div>
</div>
</form>
<div id="backgroundPopup1" class="SkipMeIAmAlradyFixPushed" ></div>
</p>
</div>
</div>
<?php else: ?>
<?php endif ?>
<?php $myStatus = Mage::getSingleton('customer/session')->isLoggedIn() ?>
<?php if(!$myStatus): ?>
<style type="text/css">
.blankdiv{background-color:#000;
position:fixed;
z-index: 9001;
top:0px; height:100%;
left:0px;
width:100%; opacity: 0.65;
filter:alpha(opacity=65);}
#popupform1{height: 100%;
left: 0;
padding: 15px;
position: fixed;
top: 0;
width:97%;
z-index: 10001;
}
#popupform1 .applyform{position:relative; overflow:auto;
background-color:#fff;
width:500px;
height:500px; margin:5% auto auto auto;
z-index: 9002; padding:10px; border:10px solid #7F3814; }
#pclose{background-image: url("http://www.americanpowergear.com/close.png");
background-position: left top;
background-repeat: no-repeat;
cursor: pointer;
height: 25px;
margin: 5% auto -6%;
position: relative;
right: -348px;
text-indent: -9999px;
top: 0;
width: 25px;
z-index: 9999;}
</style>
<div id="popupform1" style="display:block" width="500" height="500">
<div class="blankdiv" style="opacity:1!important; background-color: #FFFFFF;"></div>
<div class="applyform"><h1 align="center"> Welcome Login Form </h1>
<p id="contactArea">
<form id="login-form" method="post" action="baseurl/customer/account/loginPost/">
<div class="content">
<!-- <form id="login-form" method="post" action="http://www.americanpowergear.com/index.php/customer/account/loginPost/">-->
<input type="hidden" name="form_key" value="<?php echo Mage::getSingleton('core/session')->getFormKey(); ?>" />
<div class="clearfix">
<div class="registered-users">
<div class="content">This is a demo store. Any orders placed through this store will not be honored or fulfilled.
<div class="border"></div>
<ul class="form-list">
<li>
<label class="required" for="email"><em>*</em>Email Address</label>
<div class="input-box">
<input type="text" title="Email Address" class="input-text required-entry validate-email" id="email" value="" name="login[username]">
</div>
</li>
<li>
<label class="required" for="pass"><em>*</em>Password</label>
<div class="input-box">
<input type="password" title="Password" id="pass" class="input-text required-entry validate-password" name="login[password]">
</div>
</li>
</ul>
<div class="buttons-set">
<button id="send2" name="send" title="Login" class="button" type="submit"><span><span>Login</span></span></button>
</div>
</div>
</div>
</div>
</form>
<div id="backgroundPopup1" class="SkipMeIAmAlradyFixPushed" ></div>
</p>
</div>
</div>
<?php else: ?>
<?php endif ?>