Cookies do NOT slow down a computer, at least the first million or so that you might have on your computer!
They are not a virus or spyware, but SOME people consider "tracking" cookies to be an invasion of privacy.
These tracking cookies are "3rd party cookies" from advertising servers. They can track your movements around the web in order to deliver to you customized, targeted advertising.
It all depends on ones personal level of paranoia, whether one considers tracking cookies some kind of threat. Some anti-spyware programs do not even report tracking cookies as even a minor threat.
If these tracking cookies DO bother you AND you are using Internet Explorer for a browser, a simple change of a setting can let you permanently block all 3rd party (tracking) cookies. (Much easier then constantly removing them!) Go to tools > internet options > privacy > advanced > check-mark "override automatic cookie handling" > third party cookies > tick "block" > ok > ok.
Children are about the only people that are always nervous about cookies. Because cookies are an easy way for Mommy and Daddy to find out exactly what their darling 'lil ones have been looking at on the web.
Personally, If I happen to remember it, I will delete my cookies every 6 months or so :)
source: answers.yahoo.com
