...why your wife's system attracts more 'nasties' than yours. Her computing habits are doubtless different to yours, and they may expose her to more risk of infection than you experience. An advertisement might spark her curiousity, a 'close' button on an advertising box may be something else in disguise. There will definitely be differences in the places you visit and the way you interact with them.
Zeroth, to expand a bit on what Catweazle said here, instead of closing popups with the X (which some use to 'execute'), right-click on the ad and select Close; this may help prevent some of the problems.