Start by changing the encryption protocol on your modem / router from WEP (default - not sure why they do that as WEP is crap) to WPA2. WEP seems to screw up speeds for some people and it's also rubbish for security. ISPs might pop it on for compatibility and to save on customer support calls but WPA2 is a lot better. Make sure your computers Wi-Fi adapter supports it too if you can, though that should become obvious when you switch over to it.