|First, check your connection speed:|
Remember, download speed is generally about 1/10th of connect speed (i.e. a 1.5MB connection will usually download somewhere around 150K/s). Also cable-broadband tends to be more susceptible to "local-traffic" slowdowns; DSL is more susceptible to slowdowns due to distance from your provider.
If your connection speed isn't what you're paying for, you need to contact your ISP. Otherwise, check for spyware/malware.