In March or April 2016 Microsoft began changing something about the way it’s Windows Update service worked. Subsequently, anyone installing a clean copy of Windows 7 SP1 saw the initial Windows Update consuming 100% of one core, and running indefinitely.
Many workarounds were proposed, discovered, tested. Ed Bott weighed in with one solution. That worked for some, for a while, then stopped working. Woody Leonhard suggested a couple additional workaround, neither of which work any more (circa August 2016).
We now may have a stable workaround. At this time (again, circa August 2016) the fix seems to be manually downloading and applying these two updates, in this sequence.
After applying those patches (in sequence) to a clean Win 7 SP1 install, your initial check for Windows Updates should take less than 20 minutes and should not consume anywhere near 100% of one core.
Note: without the first patch, the second one will refuse to install. The second one contains the Windows Update fix.
I tested this with a 64-bit Windows 7 SP1 installation. Haven’t tested with 32-bit Windows 7 or with Windows 2008 Server R2 (for which both patches are available).