Haven't reinstalled Windows since April '04, and that was due to a new RAID chip on a new mobo (Athlon XP 3000+/400). Other Windows machine I did in June (Athlon 900), due to newly found hard drives (had 40GB, then added in a 20, 40, and 60, made it into a JBOD array with the onboard RAID and reinstalled on a "160"GB). Never reinstalled on the laptop (got it Jan '05). On my Linux one (Celeron 1100), which I've had for about 2 weeks working, I'd say 2 times. It crashed before install about 10 times, swapped mobo+cpu, still crashed, realized it was due to how I partitioned drives (had /boot on RAID1, put it on normal partition, then proceeded to make a RAID5 array at / -- three 20GB drives). Then, the 3 other Windows ones haven't been reinstalled in atleast 1 year (2x Celeron 1100; Athlon 1300). I have this problem where I have to use RAID, I don't know what it is. I think this adds up to 7 at my house... Note: The Celerons were free, I only bought the AMDs
As far as regular reinstall/reformats, I don't believe in them. I think that if you screw it up enough you'll have to, but its irregular. For some dang reason I don't get any viruses or malware, and I do the shadiest things without any anti-virus program. Amazing! Yeah, the registry gets larger and larger with install/uninstall, but it's not much larger, and I don't notice any performance decrease.
At work, I have Windows 2000 Server (P4 1.7 Socket 423) that I use every day and inherited from the previous worker in January (they literally just stopped showing up, no 'I quit' or anything). Best I can tell, it had been installed a couple years ago. Also have a 4 Processor Xeon with a 4 drive RAID5 SCSI and 2 drive RAID1 IDE with W2K Advanced Server that has never been reinstalled since we got it, some time ago -- the Xeons are 400MHz.