First things first. Does this happen in some other websites? If it does get a pop up blocker for your browser, or if you are using Internet Explorer do yourself a favor and install Google Chrome and install the Ad-Blocker extension, if you decided to keep your current browser be sure to clean all cookies, temp files, history, etc and then proceed to visit your blog, if it happens again then we need to step it up a notch. Spyware, malware maybe installed in your computer without you knowing it. Get SuperAntiSpyware install it in your PC update it and the let it do a thorough scan. Then do the same with Malwarebytes. Visit your site one more time and it should be OK, if it's not then there are two scenarios I can think of:
1.- Somehow unwanted code was added to your blog (maybe through a widget or app installed on it) and that you have no clue it was installed in the first place.
2.- If you are enrolled in the Google Adsense program, which gives you money if you allow ads to be posted in your blog, then maybe the popup could be a result of the program.
If all of this doesn't help I recommend you post your blogs address so we can test it and include a screenshot of the pop up in question. Also, to cover all of our bases, when does this pop up appear, randomly or at the press of a link or section of the site?
Well will be difficult, start removing your widgets one by one and see if will help. Take out the ads you added to your site.Perhaps the popup comes from Photobucket, or Motigo webstats ...If you have another pc then add a popup blocker then connect to your blog and see any messages.Sometime if someone replies to your threads then possible ther is a link there that might do this.
try Quero Toolbar
http://www.quero.at/
use another theme for your blog