Jet A1,
I can't edit the system.ini in Windows XP it doesnt have the explorer.exe prompt and it is hidden when using msconfig at the Start>run option.
Duster is not a destructive virus. It sounds like you either have other viruses on your computer and/or you had a destructive virus at one time. You can mess with your system and spend a lot of time trying to fix it, but in the long run your best bet is a fresh install. Your system has been corrupted.
Give
Trend Micro's HouseCall a run to see if you have other viruses currently on your computer. (I am wondering whether or not McAfee was disabled by a virus and even though it looks like it is scanning your system, it is in fact not doing anything.)
Take Care,
Richard