This client uses a paid version of ESET anti-virus. We reformatted and reinstalled Windows, which included installing a fully up-to-date ESET install. The slowness continued.
We disabled ESET, and like magic, the computer functioned normally from then on. Thanks, ESET! :S
Let it be known that I suggested disabling ESET, but my team had already started the nuke and pave option. When it rains, it pours.