Backdoor virus in registry

I tried to follow directions to rid my computer of a virus that is in my ‘startup’ and in my registry. All i did was ‘uncheck’ the file in msconfig startup. Now, when I boot my computer it automatically (quickly) restarts, stating that i changed the startup files. My computer, a Dell Inspiron desktop, does not give the option of F8 to get to Safe Mode… I don’t know what to do at the c: prompt (when I can get there). How do I get to Safe Mode? or someplace where I can remove the registry entries with this virus?!