I'm with the imaging and restore camp. If we're talking about an identical hardware platform, then I don't see the upside of an OS rebuild. Especially if we're talking about an HTPC here. Having to set up recordings again. Re-creating junction points. Re-configuring all my 3rd party plugins and apps and such. Not worth it.
If it's my primary desktop and I'm trying to squeeze every ounce out of it. Ok, then maybe I install fresh to get the registry cleared out and such. But we're talking about what should essentially be a black box with 99.99% uptime. What does that conversation with the wife look like anyways? "Honey, I'm going to marginally increase the performance of the HTPC. For that reason, no TV for you for the next 4 hours. And oh yeah, your existing HBO recordings will be lost."
Swap > Rebuild.