How often do you reformat?

1 - 1.5 years

mostly because I have installed so much crap that I want to get rid of.

If you know what you're doing windows itself ( XP at least ) won't require you to format
 


seperate partition for my /home on ubuntu makes reinstalling painless.

usually only reinstall when there's a new version of ubuntu with a feture / package i want
 
When I had a PC, least every 6 months to a year, sometimes multiple in a 3 month period if a new version of windows was out :P

My Macbook, only had to reformat it once since I got it in 2008, and that was because windows decided it wanted to rewrite the 'boot records' after installing it via bootcamp. Then later I upgraded to a 250GB.

And that pic comparison above, only makes sense if you need a game station (it's also much outdated in terms of what a mac can actually run in terms of games). I'll admit the hardware is tad overpriced for what it is, but where else are you going to get OSX without going thru the hassle of making a hackintosh.

For the mac users, DaisyDisk and AppZapper makes it even less likely to need a reformat.
 
here's what you do if you're on windows:

Revo Uninstaller Pro - Uninstall software, remove programs, solve uninstall problems
CCleaner - Optimization and Cleaning - Free Download
Defraggler - File and Disk Defragmentation - Free Download
Speccy - System Information - Free Download
BillP Studios - WinPatrol 18
Microsoft Security Essentials
Malwarebytes
PeerBlock ? Peerblock Site
SUPERAntiSpyware.com | Remove Malware | Remove Spyware - AntiMalware, AntiSpyware, AntiAdware!
Disk Space Fan - Download disk space management software
get avast if you don't have MSE

Most of these can be had here via a one-click background install: Ninite Easy PC Setup - Silent Unattended Install Multiple Programs At Once

Scan once a week while you sleep. CClean 3-4 times a week, as well as using the registry cleaner part. Between Revo uninstaller and ccleaner your registry will be fine. Peerblock for the torrents/random stuff, and winpatrol for when you're installing programs. Keeps your startup time low.

If you're looking for command launchers, there's http://www.humanized.com/enso/launcher/ http://www.launchy.net/ http://www.executor.dk/. I love humanized, but mozilla bought the dev team and so they don't update it anymore. Executor comes in a close second though.
 

<sarcasm>Sheesh thought we were supposed to be cleaning the computer not filling it up with more stuff</sarcasm>
 
On Linux machines, never

On PCs only when shit happens, so bad malware that even Combo Fix (the great last resort for Windows users) can fix.
 
Used to do it every year, but with Windows 7 (and XP SP3) it doesn't feel necessary anymore. Also, I buy a new computer every 2 to 3 years.