On one particular PC that I was asked to clean there were in excess of 1,100 bits of spyware, greyware, adware, malware, trojans and viruses.
I happen to know the owner personally, who is a security officer.
The infections were tracked down to his internet service provider, the provider's security was breached and a flood of crap went out to a lot of users.
Needless to say make sure you are with a big well known provider, they can afford better security for their site.
Advice, dont be a cheapskate when shopping around for an ISP