'your computer is infected' red shield, white X virus?

One of the computers on our office network caught the "your computer is infected" virus. It has a red shield-like symbol with a white X in the the sys tray and constantly pops that message. This PC didn't have any virus protection programs. Installing them now is impossible because the virus blocks them. Can I share the whole root (C) drive and attack it with my AVG from another networked PC?? Anyone familiar with this particular virus know if it is contagious to other networked PCs without direct injection (say through a shared folder). Thanks