Computer help please (upgrades)?

I want to upgrade my PC as it seems to go a little slow at times. I've done all the scans (ad-ware, spyware, system registry etc...) and it still seems to completely freeze at times. Anyway to the point, is it worth upgrading my RAM (currently 1GB) or is it worth doing the processor (AMD sempron +2800). If i change the mother board do i have to re-install all of my programs or is it just plug and play. I kind of know what i'm talking about but as you can see from my question, with my limited knowledge i really havent got a clue. As a side issue, if i have my scanner plugged in on start up i get the blue screen of death yet if i leave it unplugged until i've booted up, all works fine. I ume i am overlaoding my USB's or is it something more sinister? Any help would be welcome