Q: My Mac Pro has suddenly lost all info on the desktop plus it has gotten very slow.

try running disk Utility (in Applications folder), maybe repair permissions and repair hard drive.  I hope you have backed up all your important info beforehand (don't want to scare you, but being prepared is a good thing)

run Activity monitor to see if there is something running that is hogging system resources.  How much Ram (memory) do you have on board. Check under the Apple, about this mac. You are running Leopard, aren't you?

you may also try resetting the SMC. Shut down,  remove power cord. wait about 8 minutes. re-insert power cord. wait about a minute and reboot Mac pro. I think that's right, but not 100% sure.  If you see a blue dot underneath an icon (in the Dock, usually), that means the Application is still running, usually you have to quit it. Just because you have closed it, or aren't using it presently, doesn't mean that it'll quit on its own. I'm sure you know all about that, but I just thought I'd mention it.  Could be a problem with your graphics card, too. If you can, shut it down,  wait a bit, remove the side panel,  and check the card for dust buildup (the ATI 2600 and 1900 were very bad for this). A shot of compressed air may help here.

hopefully, some of this helps

JB

you can't repair your boot drive w/o having another boot media.

Clone your system. If you aren't running 10.6.8 you are not getting support for security.

How to clone your system:

http://macperformanceguide.com/Mac-HowToClone-backup.html

http://macperformanceguide.com/Mac-HowToClone.html

http://www.macupdate.com/app/mac/7032/carbon-copy-cloner

http://www.macperformanceguide.com/blog/2012/20120711_2-MacPro-internal-clone-ba ckup.html

Mac Running Slow: General Tips & Advice (Kappy)

https://discussions.apple.com/thread/4214422?

Repairing permissions is not going to change system being slow.

Telling us more about your system helps and is necessary.

General purpose Mac troubleshooting guide:

Mac OS X: Starting up in Safe Mode - http://docs.info.apple.com/article.html?artnum=107393

What is Safe Boot, Safe Mode? (Mac OS X) - http://support.apple.com/kb/HT1564

Isolating issues in Mac OS X

Creating a temporary user to isolate user-specific problems:

Isolating an issue by using another user account

Identifying resource hogs and other tips:

Using Activity Monitor to read System Memory and determine how much RAM is being used

Starting the computer in "safe mode":

Mac OS X: What is Safe Boot, Safe Mode?

To identify potential hardware problems:

Apple Hardware Test

General Mac maintenance:

Tips to keep your Mac in top form

OmniDiskSweeper

https://www.omnigroup.com/products/omnidisksweeper/

Recovery Mode

http://support.apple.com/kb/HT4718

TimeMachine 101

https://support.apple.com/kb/HT1427

http://www.apple.com/support/timemachine

Your Hard drive may be failing.

You need to run Disk Utility (Repair Disk), and there is a copy of Disk Utility on your Installer/Utilities DVD. Boot to the DVD, but do not Install. Answer only the "what language" question if presented, and wait a quarter minute for the MenuBar to be drawn, then choose Disk Utility off a Menu.

If you are not running at least 10.6, you should not be using your Mac on the Internet, it is no longer safe. At a minimum you should turn OFF Java (not to be confused with JavaScript) in all Web Browsers and leave it off. No Web game playing on this Mac OS.