I can confirm the slow down with Norton. Bought the package and installed it, then found I could have a shower AND make a cuppa while waiting for the mext page clicked to load up.
Everything was SO slow, I felt like throwing laptop in the bin, but dis installed the Norton and had a job finding the complaint site, but did get hold of a form which I completed and returned with the disc only, and they paid my money back into my bank account. It made me very wary of installing anything. I have downloaded the spybot one but am wary of running it, being another who is green about such matters.