Network troubleshooting applications?

Since you had arranged a cisco swith, I dont think there is a problem in switch.

Can you specify which LAN cable you use. (i.e. CAT-5, CAT 5e,CAT-6) and what is the maximum distance in your LAN.

My experience tell that with using CAT-6 D-link cable can go around 100m.

Please check following issues.

1) LAN Cable Length. (Not more than 100m for CAT-6 cable.) If more length is required, then use another switch in between.

2)If flooding happen due to virus problem then use combofix in each PC in same mode using instructions.

URL for combofix: (http://www.bleepingcomputer.com/download/anti-virus/combofix)

Mostly speed related issues happen due to spyware or virus activity. Combofix is a free and robust solution for all major virus problems.