What will happen? Over time, you'll accumulate all the same malware you had, and more, and your computer will be spewing out spam and malware to the other poor suckers out there. And it will seem dog-slow to you, because it's no longer working for you --- it's working for someone else, or, more likely, a whole bunch of someones!
If you follow my advice, that won't happen, and it won't get any slower.
It sounds like the best thing to do then is run some protection but also reformat often. What do you think of ZoneAlarm and Adaware and then running Hijack this occossionally. That would still let you run fast but with some clothes on.
Some contention about what is useful and what protections to take. I guess I'll ask my friend, the PC guru, and see what he suggests.