Some keyboards actually don't work with windows 7 you have to make sure it is supported. If these came with the computer then I would contact Acer.
One thing to try would be different USB ports.
but its pi**ed me off for years that you can plug in a usb mouse and keyboard and get to the windows log in screen needing to type a password and it wont bloody recognise the mouse and keyboard but if you press f8 or f12 or del or other keys before it hits the start of windows they work and you can browse the bios or boot menu with the keyboard
if you fix a lot of pcs where they forgot to tell you theres a password and you only have a usb keyboard available this can become a royal pain in the ass