files not fully downloaded

Hi guys, I need ur kind help, my internet provider is giving me about 60 t0 70 kb/s downloading speed but problem is that majority of compressed files(.zip, .rar) giving error " unexpected end of archive" and "corrupt file" error as these are not fully downloaded e.g if file size is 49.5 MB file will be download at 49.2 etc, on the other side some of the websites also not open completely.

kindly guide me why this is happening? for the reason of internet service? or any installed software is creating problem like anti virus or browser????

 

Its Win XP professional SP2. Mozilla Firefox 3.5 and AVG antivirus