Windows 7: Unable to Download Files

I'm having a re occuring problem with downloads with windows 7 every now and then i would be unable to download anything including streaming videos etc what would happen is nothing will come up untill it has actually finished download. For example if i wanted to download a 5mb file it would download the file before i would get a dialogue for if i wanted to save or open it same with streaming videos they would finish donwloading before playing also download managers do not work when this happens the only way i have found ti fix this is to restart the comp but this is becoming frustrating as i download offpeak and this causes problems