How can I fix Rundll32.exe error?!?

My computer keeps having the legendary Rundll32.exe error I fixed it once before but now it's happened again and I can't fix it the way I did before. I can't download anything it just keeps popping up a box that says choose which program you want to run this. I've tried everything I could find on youtube such as the cmd scan that didn't work. I don't have an I386 file so forget that. And I don't have a windows cd. Nothing is working I can't download a Rundll32 file from the web it wont let me. PLEASE HELP! I can't do a system restore and wipe my entire computer. Please if anyone knows anything else I could do it would be much appreciated!