I've formated 3 times overall. And at none of those stages I had too, I just did it to keep my computer clean.
First of all, I've had this computer for 3 years and uptil 2008 I never formated it. I was young and immature and new to this whole technology.
Once I got the internet I used to roam the internet freely not knowing what a Virus, Spyware, Malware, trojans ect.. and all such were. At the beggining of 2006, I was full of them and I had no idea to get rid of it.
One of my friends offered me a Virus/Spyware cleaner. I tried it, it said it cleaned up about 20,000 viruses and spyware. At the end I still had them, I tried NTL Netguard security (Which came with my internet package) it cleaned quite good I also ran SpyBot, Ad-Adware and still had several left.
So I tried it all again in Safe Mode. I managed to fix it, and I had left it like that for 2 years until 2008. Once I did fix it I had the biggest registry problems ever. I tried all softwares and such on download.com to fix my registry, no luck not even in safe mode.
So, again I just left it like that for another year and I formatted on 2008. And after that formated I've been trying not to mess with my registry and not install something I would uninstall. But.. I did so I formated again.