pompeys405
Sounds like your browser has been hijacked by adware, this piece of malware may have used LW as a route to get into your computer but, by the little you have written, this malware now exist on its own.
There are a number of things you can do to get rid of this "pop-up".
1) Download, install and run Ad-Aware:
Ad-Aware by Lavasoft - Antivirus software, free spyware removal, firewall
2) Also download, install and run Spy-Bot:
The home of Spybot-S&D!
And if these two don't get rid of your malware (pop-up) then you are going to have to delve deeper with "Hijack This" but you will need help to use it.
Trend Micro HijackThis - Free software downloads and software reviews - CNET Download.com
UK Bob
PS. All the above spyware removers are free.