Due to human nature I concentrated on your thread title, and interpreted/equated, not available as cannot enable.
You probably have a corrupted/crippled install.
If you have just the one instance of google installed, you will have to remove it, and then re-install it.
If you have problems deleting it, it maybe because it is the default.
For the time being switch to bing as the default, delete the google entry. Then do the re-install.
You may want to try using IE's version: Internet Explorer Gallery | Add ons | Google Search
For an explanation: Remove older crippled Google Search Provider
It is for IE9, but I would think the same affect would happen to IE8.
Scroll down to the second screenshot, he explains that the crippled google had a .gr suffix instead of .com that is why it was crippled.
Even if yours does have .com I would still get IE's version, sometimes the two do not play nice when they intermingle, ever notice you can only download updates from microsoft with IE? Other browsers do not work.