for all the crap websites put on your computer ( like spyware, toolbar and all that shite - they should be prosecuted for invasion of privacy but anyhow ) can try a program called ad-aware which will find many spyware programs and remove them.
It doesn't remove all though as some creep into and make changes on the registry so you need some program that will find those and remove them. I use a program called "registry mechanic" which is quite good.
Try those two
Queenbee and your computer will be releaved
For codecs you need the wmp 8,9 codecs, Divx, and Xvid as well as a DVD playback program that will install the mpeg-2 codec. That's all I use and they're sufficient. Those codec packs like Nimo are not recommened on the basis of being particularly crappy.