I bet you use Internet Explorer to browse the internet. It's the reason for most of the spyware since the malware writers are too familiar with it.

" Some of the really sleazy parasites, particularly homepage-hijackers and diallers, execute by exploiting security holes in Internet Explorer, ways of getting code to run that are not supposed to be possible, but are due to mistakes in the browser code." and more...
http://www.doxdesk.com/parasite/

If you really want to cut down on the items coming in, just use Firefox or Mozilla when you can.

Bob