$hf_mig$ is missing.

I think CCleaner may have deleted it...if CCleaner is still doing this. Anyway, what problems would this have caused my computer, and more importantly, how do I get it back?

Did something tell you $hf_mig$ is missing? Not exactly something I would go looking for....
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My own research gave this:

Its name $hf_mig$ stands for "Hotfix Migration". It's for tracking versioning information about hotfixes to keep from accidentally breaking or downgrading your system. Hence, you definitely should not delete it. However, if you don’t ever plan to roll back any hotfix or service pack, it’s OK to delete them.
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How I suspect someone gets it back, is uninstalling all the service packs and hot fixes, and letting Windows Update redo them.

If Norton told you, tell him I said I wish him harm as much as he has caused computers over the years.

Good info, Faladu, thanks. I guess I'd like it back, but I do have many reservations about doing what you are suggesting. But, let me also say, I know nothing about services and such, however my gut quivers at the thought of deleting a lot of stuff and then letting Windows rebuild.