Not sure if this is the right place to ask this question. But I have a router which connects to the internet. Now I have a switch connected to this router. I added a lot more computers so I added another switch and connected it to the first switch using a cross-over cable.
As soon as I connect it to the first switch, my lights in first switch start blinking like crazy and my entire network simply stops working. The minute I remove the second switch's wire, its all fine again.
What could be the problem?
This sound suspiciously like you have the switches plugged into eachother in a circle. E.g. Switch A is plugged into Switch B which is plugged into Switch C which is plugged into Switch A. So broadcasts go A -> B -> C -> A -> B -> C -> A -> B -> C so on and so on at the maximum speed the switch will support, thus rendering the network unusable.
Make sure that none of the switches are plugged into any other switches except for that central switch (the router. Which I'm guessing is a consumer grade router that also has a switch in it).
Edit: Just saw your comments about the brands of the products. For clarifications sake, you actually have three switches in your scenario (your DIR-300 has a router AND a switch. It's the switch that's causing the issue here).
Usually when I connect switches, I just use a regular cable because most of them switch automatically, although they should understand a crossover as well.
A few possible trouble shooting possiblities, if you haven't already tried:
If you're using a crossover cable make sure you're not plugging it into an uplink port on the switches.
Connect modem to router via uplink port then run a crossover cable from 4 port switch in router to switch1/portX, then run a crossover cable from switch1/portY to switch2/portX then plug in devices as needed.
I would suggest that you use the older switch for the lest amount of traffic and/or as the switch2 in this example.