Wallpaper pix took over computer like a serious PC killer virus...

I am running W98se systems in several old PC's.. experimenting with junk HD's... In this one machine, I downloaded a wallpaper pix from "desktopgirls.com"... When I clicked programs, it read "empty"... On the next reboot the icon's pix vanished... On the the second next reboot the icon's words vanished.. then all I had was the wallpaper pix as my desktop.. and popups that said something about the PC starting in standby, and ms something, and explorer to be shut down...

Now all I get on a reboot, is an instant mouse cursor arrow-icon in the bottom left of the screen... I can't even format C...

How can I get into the front-end controls, enough to format C ?..