gregg, you can review the Microsoft End-User License Terms for your OS, which includes warranty info, here:
In the terms for OEM installs of Vista, I find these statements notable:
"If you follow the instructions and the software is properly licensed, the software will perform substantially as described in the Microsoft materials that you receive in or with the software."
"The limited warranty covers the software for 90 days after acquired by the first user."
"This warranty does not cover problems caused by your acts (or failures to act),..."
I expect that "your act" of using Stardock or other tweaks/hacks would exclude you from warranty coverage if your OS no longer performs "substantially as described". The XP terms are similar. Also, if your 90 days are up for an OEM install, go nuts because you have no warranty anymore. Also, Windows 7, being a Release Candidate, has no warranty so hack away!