I have some experience to tell you from the last days.(from my neighbours pc problem with the DSL speed)
A fresh Windows with IE.
no antivirus software or any antispyware blockin software or a firewall.
The Results are 36 viruses/trjoans/worms some very common things like blaster mydoom and some others.
Thats wasnt all a shitload when you look at the upload and the result is you cant do anything with ie.

And you know what ? That all in just 15mins
Same System with firefox and you are somewhat saver