1. check if the slowness is experienced at other computers or just at yours? at all times of the day ? at all web sites ?

2. the slowness might be caused by malware (viruses, trojan horses) that is using bandwidth and processor resources;
Go to  [control panel] > [security center] and make sure the firewall, automatic updates and virus protection are all "ON"

3. Make sure your computer (and, if possible, all others) are running an anti-spyware program such as ad-aware, spybot and microsoft windows defender

4. follow the  recommendations at:   http://www.firewallguide.com/spyware.htm 

good luck !