I would like to recommend you to upgrade your Windows XP operating system to a more modern one, such as Windows 7 or maybe Windows 8 because Microsoft discontinued support for Windows XP on April 8 of this year.
If the upgrade isn't possible at this time, there are a few steps you can take to remove the infection. Security Essentials will not be receiving anymore updates and it's actually not available for download on WinXP anymore, that is a possible reason for it being ineffective in removing the virus. I would recommend at this point to try and do a scan with Sophos Virus Removal Tool, after you download it, it will need to be updated before you scan. If Sophos doesn't work in normal mode, like wpgwpg stated with other AV scans, try it in Safe Mode as well.
After Sophos has finished, download and scan your machine with Adwcleaner, it will need to restart the machine after it finishes scanning and removal. The last thing I would do is to perform a full system scan with Norton Antivirus, if you don't own Norton Antivirus you can download a 30-day trial from their website and you will be able to conduct a full virus scan. I encourage you to purchase the software if you like it.