Check it, Yahoo (and everyone else I reckon) uses something called "web beacons" to track all the sites you click on even if your not on a yahoo page, and its specific to each machine, not each account, so every machine you have, you need to opt out of their web beacons,, fucking bastards...if you can't figger out how to opt out, gimme a shout, there are a couple dif ways to do it
http://info.yahoo.com/privacy/us/yahoo/webbeacons/details.html