First, I've seen many computers that slow down
because they have norton running. Between the antivirus scanning every file that is modified on the comptuer and things like the firewall and spyware protection constantly running, it can quickly bog down a system. However, if you say you disabled all startup items, I assume that includes norton, so that's probably not your problem.
One possability is that you have a virus or spywre which has installed itself as a rootkit. A Rootkit modifies the way your computer access teh hard drive and memory so that is is almost completely hidden. You can detect (although not remove) rootkits using Sysinternals' free
RootkitRevealer.