i need help .. my router ..

[free4eveeer]

plz guyz i need ur help (i'll try give most all the details here) .. i have a wireless 3com router connected to 512 kb/s DSL .. we are four computers connected to its 4 ports .. 2 PCs are connected with 100 meters of CAT5 cable .. and the 3rd 50 meteres and the 4th only less than 10 meteres .. the problem is when one of the users downloading anything with even only 15 kb of the whole 50+kb we have all other PCs just dies .. we cant even open a website .. i put Netlimiter 2 on my pc to c whats going on .. and i find internet explorer takes like 15 to 20 seconds before it even start to load .. and when it does it takes 2 or 3 seconds and rests again and so on .. i may stay like 2 min. to open a website . also when im downloading with bitcomet and my PC is the only one in the network i limit it to 5 kb down and 5 up .. also it kills my connection and i cant open anything .. im not sure if its a router problem or cables one .. i thought of buying a switch or something .. plz i need ur advice guys and dont hesitate to ask for more details .