Any anti virus scan will slow down other use of the computer, but another good reason to ditch Norton or Mcafee and use (the free!) microsoft security essentials is you can set a limit on the maximum CPU percentage to use during scans in MSE so you can minimise the effect a scan will have on you.
Sorry, I got that wrong. I know I've got Microsoft in my pc, but is this free security in already, I know microsaoft automatically screen emails etc., so is it part of that?