Problems with internet

My uncle is the one in charge of providing internet service at the office. He hires a residential adsl, 256/128 and offers it at my parents office. My parents lease individual rooms for business use (a very common thing to do in Mexico, don't know if in the USA also).

He uses a Trendware router, 2 Linksys Access points and 4 Linksys USB antennas. The reasons of 2 AP, becuase the walls are from a material that is 30cm lentgh, another AP was needed to reach the other end of the building.

Something like this

Adsl modem
|
Router-->AP-->2 USB antennas
|
|-->AP--> 2 USB antennas

My father has 1 USB antennas and my mom, new guy(below) , and I connect through my fathers computer with ICS.

Latelly the internet comes and goes every 5 seconds, you can't even load 2 images at the same time! Coincidence that the problems occured when the new guy arrived, but I made a fast checkup on his machine, disabled startup programs he might have that use internet resources. Ran Ad-aware 6, he also has NAV corporate 7.6 (updated) and Win XP PRO but still the same problem.

When I was at school my uncle made some troubleshooting, disconnect the second AP (where my dad is connected) and told me that the internet worked great, but I don't trust him. He is not what I call, a reasonable person that analizies everything. Maybe it did work for a while, but he didn't test it for a couple of hours.

If I disconnect the power cable from the modem and router, internet speeds are good, but go down after a few minutes.

Can it be a virus or something that is using all the resource?

I have access to 3 machines with the USB antenna (father's, parent's secretary and uncle). The other I don't, I would need a very good explanation to do troubleshooting on the machine.

any ideas?, please! I'm getting desperate.
BTW, my uncle believes I'm just a kid who plays games all day :rollseyes: