Internet Connection will only work on Safe Mode

I lost my internet connection on my other computer. It only lets me access internet, if I boot to safe mode.
My security software is turned on, so I don’t think it’s because of a virus or Malware. It is one of those old Dell Optiplex systems and not under warranty anymore. So I cannot get support from their tech support without paying a fee for support.
What else can I do, aside from system restore and OS reinstallation? I don’t have the OS CD anymore at hand.
Please help me if you can. Thank you so much in advance.
