If you see competitors using your trademark in their copy on MSN.com or Live.com, you can use this form to report it. Make sure to record the confirmation number after submission. Kind of a lengthy form, so make sure to have your TM registration number handy, as well as the keyword(s) and ad(s) that are in question.
I have just submitted a request for one of our clients, and will update this once I hear back.
Also, If you are in a highly competitive space and you have other companies bidding on your TM terms, or worse yet using your TM terms in their copy, you should check out AdGooroo’s TM monitoring product. It monitors both keyword bidding and ad copy incidents on Google, Yahoo & MSN.