Windows 7 sleep while program running?

I have Windows 7. It's great in most ways but really dumb in others. It goes to sleep while the computer is downloading (in Firefox), decrypting/encoding a DVD (DVD Decrypter), and running programs like Spybot. This ain't good 'cause the downloads fail and defeats the purpose of doing something else while scanning or decrypting/encoding. Anyone else have this problem and how to fix it other than turn off sleep mode? Thanks.