BleepingComputer.com: auto disconnect when...

im using a modem given free by my isp.its called riger db108-wl wireless modem.im using a desktop with windows 7 ultimate 32-bit.
my problem is,whenever i visit a file hosting site to download a file,ex:www.hotfile.com then my internet is disconnected.since this is a wireless modem,all pc connected get disconnected as well.but when i ping any website with cmd theres still response.then i tried using a vpn software(hotspot shield).with this software activated i can connect to internet while other laptop which don't have it cant connect.so im asking is this related to isp?sry my english is not very good