How do I fix my computer if it won't let me download anything from the...

...internet or CD ROM? My computer has malware on it, and I think I accidentally might have deleted an important process or system. I don't know much about computers but can tell you that whenever I try to download something, or when attempting to open many of my files, I'm not given access and am presented with an error message usually stating that "rundll32.exe" cannot be found, and sometimes other exe's instead. I've already tried to take advise on the internet and got as far as accessing regedit and adding a folder called "rundll32.exe" under the first listed "HKEY ..." but do not know which I should add it under or what I should make the "data value" - or if I'm even taking the right approach. Please, if you know about computers, throw me a bone, Thanx