Not sure what you mean by "activate". Once you boot, see if Windows found a new drive, i.e. look in My Computer for it. Windows should automatically find new drives if they're installed, and it should automatically assign them a drive letter and such. If Windows found the drive you just have to format it, then you can use it. Right click on it and pick Properties or whatever look around in that window, an option for formatting is probably in there somewhere.

If Windows hasn't found the drive, see if your hard drive came with a CD or disk and use that.