Here is my problem and any help to solve this issue will be highly appreciated.

I live with my friends and all total there are 4 laptops connected to the internet thru my wireless router. Lately, the internet has become very slow and we are not able to figure out the reason behind this. The ISP provider comcast says that they are not able to find any issues. I want to know if there is any tool or something which can tell me how many computers are connected to my network at any point of time and also the network usage of each of them.