Using another computer, download one of the free AV programs like AVG or Avast and load it onto a memory stick. Then try to install on the affected machine.
If the virus still rejects the install, start the machine in Safe Mode (press F8 as Windows starts) and try install again.
You might also download Easycleaner which will inspect programs intended to Start automatically and may allow you to stop the virus starting again.
Identify some key files involved in the virus (sometimes the names will come up in error messages as you start the machine or when the virus tries to access the internet with that disconnected) . Once you have some file names try Googling them and you may find sites which will identify the virus and guide you on eliminating it (better than I can).