Computer keeps locking up when downloading?

My computer had been working fine until about a month ago. Now, whenever I try to download anything, such as a game, antivirus software, a movie from Netflix, or even listen to a CD, it works for a little while, but then locks up completely. I can't use Cntrl-Alt-delete, or anything else. The mouse doesn't work; I have to turn the power off manually and restart the computer. I'm able to play the games on Pogo, but that's about it. Does anyone have any idea what might be causing this. I cleaned the fan already, and it didn't make any difference. I would appreciate any ideas of things I can do myself to maybe figure this out, as I can;t really afford to take it for repairs at this time. Thanks in advance.