stop downloading files for microsoft isa server 2000 clients

I have network of 45 users, and I have 512k link for internet browsing. I have ISA server 2000 for internet sharing as an integrated mode. I want that my client could not download any file, but I want them only to surf/browse internet. Please help me in that my clients could not download files. I will be very thankful to that person how will help me in that.