Desktop: XP Home, reinstalled once after major hardware change about a year ago.
Laptop: Vista Home Premium, reinstalled once as soon as I got it home. OS had a faulty factory install, been fine since reinstall.
Previously, on various systems: Windows 98, several times. Fedora, at least a few times. Windows 95, several times. DOS 6.22 and Windows 3.11... too long ago... more than a few times, much more.
Girlfriend's laptop: Windows XP Professional, 3 times in 3 years. Speaking of which, it's getting time for her to break her computer again.