Check for spyware using the free versions of both these tools and let them delete anything they find. (Spyware can prevent security software from working and even stop you accessing the webpages associated with it). http://www.superantispyware.com/superantispywarefreevspro.htmlhttp://www.malwarebytes.org/mbam.php
Your XP should be brought up to date with SP3 tio be fully supported, there's help with that here: http://community.mcafee.com/showthread.php?t=220807
You may be using IE6 still, I suggest installing IE8 from Microsoft directly: http://www.microsoft.com/windows/internet-explorer/default.aspx
Plus make sure you have the latest Flash, Shockwave and Java. http://www.adobe.com/shockwave/welcome/ http://java.com/en/download/index.jsp
McAfee's webpages and applications rely on an up to date and working IE, so go to IE Tools/Internet Options and do some maintenance.
If IE6: General tab - delete browsing history, Security tab - make sure all zones are at default, Contrnt tab - make sure content advisor is off.
If IE7 or 8: Go to the Advanced tab, click Reset, then Apply and OK.
Close and reopen the browser and then go to Tools/Manage Add-ons and re-enable the add-ons.
Try again for your account at https://home.mcafee.com/Secure/Protected/Login.aspx