Pop up logout button - xpath

Question

In my application, logging out is a two way process. When you click on the Logout link on the main screen, it opens up a pop-up for confirmation of logout.
Now, on pop-up, there are two button ( 'Logout' and 'Cancel'). The problem is that I am not able to click on either of them.
Using Firebug console, I confirmed that the XPath which I am using is correct. But when using the same Xpath in webdriver, it's not able to locate the object.
Xpath of the button on the pop-up : //div[3]/div/div/div[2]/div/a/span
Code which I am trying :
IWebElement LogoutButton = WebDriver.FindElement(By.XPath("//div[3]/div/div/div[2]/div/a/span"));
LogoutButton.Click();
Is there any specific method to use here ?? Please let me know.
cheers
EDIT*
<div id="ext-comp-1084" class="x-window x-layer x-window-default x-closable x-window-closable x-window-default-closable x-popup" tabindex="-1" style="opacity: 1; width: 250px; height: 145px; right: auto; left: 755px; top: 147px; z-index: 19000;">
<div id="ext-comp-1084_header" class="x-window-header ext-header ext-header-horizontal x-docked x-unselectable x-window-header-default x-horizontal x-window-header-horizontal x-window-header-default-horizontal x-top x-window-header-top x-window-header-default-top x-docked-top x-window-header-docked-top x-window-header-default-docked-top" style="width: 250px; right: auto; left: 0px; top: 0px;">
<div id="ext-comp-1084-body" class="x-window-body x-window-body-default x-closable x-window-body-closable x-window-body-default-closable x-window-body-default x-window-body-default-closable" style="width: 250px; left: 0px; top: 52px; height: 49px;">
<div id="toolbar-1086" class="x-toolbar x-docked x-toolbar-default x-docked-bottom x-toolbar-docked-bottom x-toolbar-default-docked-bottom x-box-layout-ct" style="width: 250px; right: auto; left: 0px; top: 101px;">
<div id="toolbar-1086-innerCt" class="x-box-inner " role="presentation" style="width: 250px; height: 44px;">
<div id="toolbar-1086-targetEl" class="x-box-target" style="width: 250px;">
<div id="tbspacer-1087" class="x-toolbar-spacer x-box-item x-toolbar-item x-toolbar-spacer-default" style="right: auto; left: 0px; margin: 0px; width: 50px; top: 22px;"></div>
<div id="button-1088" class="x-btn green x-exclude x-unselectable x-box-item x-toolbar-item x-btn-default-toolbar-small x-noicon x-btn-noicon x-btn-default-toolbar-small-noicon" style="border-width: 1px; width: 100px; right: auto; left: 50px; margin: 0px; top: 8px;">
<div id="button-1088-btnWrap" class="x-btn-wrap" unselectable="on">
<a id="button-1088-btnEl" class="x-btn-button" tabindex="0" unselectable="on" hidefocus="on" role="button">
<span id="button-1088-btnInnerEl" class="x-btn-inner x-btn-inner-center" unselectable="on">Logout</span>
<span id="button-1088-btnIconEl" class="x-btn-icon-el " style="" unselectable="on" role="img"></span>
</a>
</div>
</div>
<div id="button-1089" class="x-btn gray x-exclude x-unselectable x-box-item x-toolbar-item x-btn-default-toolbar-small x-noicon x-btn-noicon x-btn-default-toolbar-small-noicon" style="border-width: 1px; width: 80px; right: auto; left: 160px; margin: 0px; top: 8px;">
<div id="button-1089-btnWrap" class="x-btn-wrap" unselectable="on">
<a id="button-1089-btnEl" class="x-btn-button" tabindex="0" unselectable="on" hidefocus="on" role="button">
<span id="button-1089-btnInnerEl" class="x-btn-inner x-btn-inner-center" unselectable="on">Cancel</span>
<span id="button-1089-btnIconEl" class="x-btn-icon-el " style="" unselectable="on" role="img"></span>
</a>
</div>
</div>
</div>
</div>
</div>
</div>

Answer 1

1) First save the instance of main window using below code
String mainWindow = driver.getWindowHandle();
2) //code to click on the Logout link on the main screen
3) then switch webdriver control to pop-up using below code
for(String childWindow : driver.getWindowHandles()){
driver.switchTo().window(childWindow);
}
4) //code to click on pop-up, ( 'Logout' and 'Cancel').