Restoring Factory Settings to get rid of viruses

We have a dell xps desktop - Windows Vista 32 bit.  We have been trying to clean this computer for four months now.  I would like to restore the factory image settings and then upgrade it to Windows 7.  My question is, when I do a restore of the factory image settings, will that finally rid my computer of the virus or will it migrate into the new restored settings?  Otherwise, I will need to format complete and do a custom install of Windows 7, which will cost me a lot more.  Thanks for your help!