Internet explorer...
Bit off topic...
I just tossed a new computer together for my brother, nice unit AMD quad core, 4G memory, 500g Sata hard drive, not bad for just over $200. Anyway I installed firefox and did all the updates, and I asked him how it was running today. He has NEVER used IE, yet he is getting IE pop-ups of ads all the time. He said he's using Kaspersky AV. Ads are anything and everything, not sex ads...
I hate IE....