I recently wiped my hdd, installed my OS and used an out of date copy of AVG to go online to update the AVG and add Spybot search and destroy, AdAware, Cleaner, Asquared, Sygate, and Ewido, and before I had finished the AVG update I was getting pop ups. Long story short is that in that short period of time I managed to get five different trojan horses, as nasty form of adware called look2me which generates lots and lots of pop up ads, and a list of other nasty stuff. But after getting all of these downloads and updates completed I was able to go into safe mode and run all of my scans, and was able to get rid of most all of them on the first scans.
If you do this, run the Ccleaner first to straighten the registry, and then clear your cookies and online history, and then start anywhere you want and proceed, I did have two tiems that Spybot requested that I have them look for them after a reboot, but I was able to rid myself of all these...I now have a clean machine. You might want to try this yourself, I found a free download site that has all of the above, if you are interested go here.
All of the programs are listed alphabetically.