Okay, take a ROM that has a kernel (boot.img)
Lets say Mobsters. Download it. Take the boot.img and put it in the sdk-tools folder (You should see a file named fastboot. If so, you're in the right folder.)
Reboot into recovery, flash the ROM over.
Once its done, open up a command propt (Start > Search > CMD) and navigate using the cd command to get there. For example, mine is in the C:\ drive. So I type:
cd C:\Android\sdk-tools
Then type in adb reboot bootloader
Once you see that, you SHOULD see the phone saying "fastboot usb" on top. If it does, you're set. Type in:
fastboot flash boot boot.img
Let it do its thing, should take a few seconds.
Then:
fastboot reboot