Ever run into an error message like this?

modprobe loop FATAL: Could not load /lib/modules/2.6.16-xenU/modules.dep: No such file or directory

If you take a peek into the /lib/modules, what do you see? Do you just see this?

2.6.21-1.3194.fc7

You can just simply create a symlink of 2.6.16-xenU to that to stop the error messages:

cd /lib/modules
ln -s 2.6.16-xenU 2.6.21-1.3194.fc7