slow connection to internet

i am a junior support in school, recently all users in my school connect to internet with IE brower very slow, I don’t know why? at the begining, i think the ISP side have something wrong, afterward, i phoned to ISP and check the connection with my school router. the answer is in good condition. So i think it is internal problem. we have a PDC and a bdc which installed netscape proxy server, i only find that the c drive in bdc is full, is it the reason of slow speed to browse website through internet or any reason will affect this occurs.and how can i slove this problem. could some experts slove my problem.
thank a lot!!!!!!!!