Me: bought off ebay? then take the DVD OEM discs and clean system install.
Upper right corner "About this Mac..." More.... takes you to System Profiler.
Top lines of what model etc. do not include serial number but you might want to call and / or check that too and order replacement DVDs if it didn't have those.
but for now, Wipe the drive.
You can backup first if you want.
Utility folder: Disk Utility: Restore: allow drag and drop the source and target.
www.macupdate.com "backup programs" I recommend Carbon Copy Cloner to maintain bootable backups and can also archive changes. Similar to what Paragon Manager Suite offers in Clone OS to SSD.
Boot from the OS X DVD it should have come with. Choose the Utilities from menu bar : Disk Utility to repair or format. Then go back and Install OS X.
Using Cloning as a Backup Strategy
http://www.apple.com/support/lion/installrecovery/ Create an OS X Lion Install dischttp://reviews.cnet.com/8301-13727_7-20080989-263/how-to-create-an-os-x-lion-ins tallation-disc
Isolating Issues in Mac OShttp://support.apple.com/kb/TS1388
Get Help with your Product
TimeMachine 101 https://support.apple.com/kb/HT1427
Mac OS X & Mountain Lion Communityhttps://discussions.apple.com/community/mac_oshttps://discussions.apple.com/community/mac_os/os_x_mountain_lion?view=discussio ns
General purpose Mac troubleshooting guide: Isolating issues in Mac OS XCreating a temporary user to isolate user-specific problems: Isolating an issue by using another user accountIdentifying resource hogs and other tips: Using Activity Monitor to read System Memory and determine how much RAM is being usedStarting the computer in "safe mode": Mac OS X: What is Safe Boot, Safe Mode?To identify potential hardware problems: Apple Hardware TestGeneral Mac maintenance: Tips to keep your Mac in top form
First, if at all possible make a backup of the drive in its present configuration. If something goes wrong during repair attempts and makes things worse you will have a backup. You will have to decide how to work this in the light of any present backups you may have, for example, a backup that may be a few days out of date. In that case you may want to keep that one and make a second backup of this as they are now, though I realize people often don't have a lot of empty drives sitting around.
Boot from the System Installer disc that came with your computer or is the one for the version you currently have on there, select language if applicable, choose utilities, run Disk Utility and verify (and repair if necessary) the drive. You can verify a drive from DU on your main drive while booted but I have found this can result in incorrect reporting of errors. To repair your drive you have to run it from a drive other than the boot drive anyway. Next, boot from your drive in Safe Mode (http://docs.info.apple.com/article.html?artnum=107393) and repair permissions. You can repair permissions while booted from the installer disc but this uses the permissions configuration on the installer disc which may be out of date if you have run any updates on your computer. Booting your computer to Safe Mode restricts the number of things running on your computer while permissions are being run and does a bit of spring cleaning at the same time.
Resolve startup issues and perform disk maintenance with Disk Utility and fsck - http://docs.info.apple.com/article.html?artnum=106214Using Disk Utility in Mac OS X 10.4.3 or later - http://docs.info.apple.com/article.html?artnum=302672Disk Utility's Repair Disk Permissions 10.0-10.6 - http://docs.info.apple.com/article.html?artnum=25751"Try Disk Utility" (modified from http://support.apple.com/kb/TS1417)
1. Insert the Mac OS X Install disc that came with your computer (Edit: Do not use this disc if it is not the same general version as what you have currently on your computer, e.g. use a Tiger disc for a Tiger drive, not a Panther disc), then restart the computer while holding the C key.
2. When your computer finishes starting up from the disc, choose Disk Utility from the Installer menu. (In Mac OS X 10.4 or later, you must select your language first.)
Important: Do not click Continue in the first screen of the Installer. If you do, you must restart from the disc again to access Disk Utility.
3. Click the First Aid tab.
4. Click the disclosure triangle to the left of the hard drive icon to display the names of your hard disk volumes and partitions.
5. Select your Mac OS X volume.
6. Click Repair. Disk Utility checks and repairs the disk."
Disconnect the AC power cord.
Press the Start button, and hold for a quarter minute.
Release and wait a quarter minute.
Plug in the AC power and wait a quarter minute.
Keyboard must be plugged into a USB on the chassis, not on a Hub or display.
Hold down the Alt/Option key as you Restart.
This invoke the All-in-ROM Startup Manager.
This draws a gray screen, then in slow motion, draws an Icon for each potentially-bootable Volume.
When all are drawn, the "eject" key (upward pointing arrow with underline) on the keyboard can be used to open the CD tray.
Insert the Installer/Utilities DVD that came with your Mac Pro, select it, and proceed.
Boot to Installer/Utilities DVD:
Boot to the Installer/Utilities DVD, but do not Install.
Answer only the "what language" question, and wait a quarter minute for the menuBar to be drawn.
Choose Disk Utility from the Utilities menu.
Select your boot Hard Drive by Hardware_name and check the SMART Status in the lower right.
Choose ( Repair Disk ).
> Tell us if anything in the above procedures did not work as described.
EDIT: The hatter's post was no there when I began composing, so my post is NOT a commentary on any suggestions he has made.