There could be several reasons for this but the most likely is that you are trying to install an unsigned driver. (If the computer is on a corporate network or under the control of a domain, the admin may be blocking it, talk to them) or even AV software.
I would first check to see if there is a "real" Windows 7 driver for the device from the manufacturer.
But f you want to try to force the install. Search this site for "Install unsigned driver".
There are several options, some temporary, some permanent, I couldn't pick like the "perfect" solution to paste here, so just do the search.
If you don't think it's because of an unsigned driver, go into more detail about exactly what device, driver your installing and what the actual error messages are...
