I used to work at my university as a helpdesk technician.
The most likely way to junk up your computer online, from what I've seen, is to use your computer as a pr0n and warez receptacle.
Just browsing normally will give you some adware issues, but usually nothing serious.
Oh, and using AOL doesn't help, either.
I think AOL is pretty much designed to muck up a system
But back to the topic...
So glad something is being done about spam mail. I can barely make myself check my hotmail account any more. For every one actual email I get (usually an Abandonia PM notification nowadays, hehe), I get about 50 junk mails.