How to format my computer without affecting the software installed?

Question

how do I format my computer without affecting the software installed?

Answer 1

You can, but if you don't format the drive using the Zero Data option all your personal files will still be accessible to someone who knows how to gain access to them. If that's not a concern then here are two options (easy and not so easy):
The Easy Way
1. First, boot from your Tiger DVD, select your language and click on the Continue button, then select Disk Utility from the Utilities menu, select your boot hard drive, go to the First Aid tab, and click the button for Repair Disk. Repeat this until no trouble is found. Then click on the Repair Permissions button. Then quit DU, return to the installer and shutdown the computer for a couple of minutes.
This will assure that the drive is OK and permissions are OK.
2. Next, boot to single user mode by restarting and after the chime press and hold down the COMMAND-S keys until a black screen with white type appears.
3. At the prompt, type the following commands pressing return after each command line:
/sbin/fsck -yf
mount -uw /
rm /private/var/db/.AppleSetupDone
shutdown -r now
The second-to-last command above will cause OS X to think that the operating system is newly installed, and when you reboot, it will send you to the startup wizard where you can start a new user without reinstalling.
The Not So Easy Way
Follow these instructions step by step to prepare a Mac for sale:
First, back up the data:
1) Shut down all Virtual PCs. They cannot be in their "fast saved" state. They must be shut down from inside Windows.
2) Clone to an external drive using Carbon Copy Cloner.
Next, prepare the machine for the new buyer:
3) Deauthorize the computer in iTunes! Deauthorize both iTunes and Audible accounts.
4) Remove Open Firmware passwords
5) Turn the brightness full up and volume nearly so.
Install a fresh OS:
6) Insert the OS X install CD/DVD.
7) Restart the computer while holding down the C key to boot from the CD/DVD.
8) Run Disk Utility from the file menu and erase the internal hard drive (optionally zero all data).
9) Install OS X.
10) Reboot the computer.
11) From the welcome screen, you can skip the registration step by typing command-Q.
12) When prompted, create an account (it will be an admin account).
13) From your new admin account, configure networking.
14) Then use Software Update to bring your system and all of it's applications up to date.
15) From Disk Utility, repair permissions on the new volume.
Now delete the account you just created:
16) Boot from a different volume (e.g. a firewire drive, if available)
17) Clean up the image using the following terminal commands:
prompt> rm /Volumes/<imagevol>/var/db/BootCache.playlist
prompt> rm /Volumes/<imagevol>/var/db/volinfo.database
prompt> rm -r /Volumes/<imagevol>/var/vm/swap*
18) Now you can get rid of the admin account you used to set up the machine Use the terminal:
prompt> nicl -raw /Volumes/<imagevol>/var/db/netinfo/local.nidb -delete /users/<admin>
prompt> rm -r /Volumes/<imagevol>/Users/<admin>
prompt> rm /Volumes/<imagevol>/var/db/.AppleSetupDone
19) Shut down and ship it to your buyer. When they get it, it will boot to the Welcome screen just like a factory Mac, except that it's better because it's completely up to date.