How do I get rid of this virus?

My PC was infected with a virus when I stupidly opened an email that purported to be from UPS. I can no longer operate the PC without buying software recommended by the virus. In safe mode I have managed to download anti-virus tools but they have not been effective. Kaspersky's avptool did not remove the virus and the avg tool could not be installed because the virus blocked it. I would prefer to avoid re-installing windows so does anyone have any other ideas?