Then follow these steps to make sure you're getting a perfect Clean Reinstall - Factory OEM Windows 7
(same for retail).
You should not be getting a windows.old folder if the drive was actually wiped since there is no installation to bundle into it. So if you got it then it wasn't wiped. Use Clean command.
If you wiped the HD with zeroes and again with Clean and malware persists you should study the actual malware discovered as it may be a rare BIOS infection which requires reflashing the BIOS chip. Sometimes even that is not enough.