Windows (like all other Microsoft teams) had a huge test team at the time — equal in size to the development team in some cases — and larger than any other company I’ve ever worked for.
Each functional team (kernel, file systems, networking, shell, etc.) had their own test teams; there were also cross-functional teams that covered overall functionality (security, UI, System Test, Stress Test, Performance, Scalability, installation and upgrade, etc). There were also teams responsible for helping partners test their solutions and get them certified.
Most of the problems with Vista had to do with partner readiness — both for applications and for device drivers. The architectural/security changes were too big and required a lot more work from partners than was typical of Windows releases.