What's the deal with this ad.doubleclick.net thing?

My computer has been running sluggish for a little while even though I ran scans on it with Spybot Search and Destroy, AVG free, Ad Aware, Superantispyware and Spyware Blaster. I have noticed ad.doubleclick.net showing up on the bottom bar whenever I log into my mail account. I added it to the sites to be blocked in my browser options and ran another program called PARETOLOGIC. It came up with 5 issues none of my other programs have detected. What's going on? Any ideas?