Q: power mac pro slow

Did you try Resetting SMC as well as NVRAM?

you know how you need to have another boot device to Repair disk drive?

That a Mac Pro and PowerMac are mutually exclusive.

If you don't have Lion or later then you don't have built in recovery partition but you can use DVD or create an emergency boot volume (30GB) and clone your system.

There isn't enough to give a quick fix other than Safe Boot.

You probably need to move t he system to another drive and assume the one you use is corrupt directory and bad sectors

And what you have installed and running or corrupt files.

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

If this started happening without your having made any changes and without a warning of low disk space, then it's likely that the boot drive, or some other hardware component, is failing. Back up all data immediately, then run the Apple Hardware Test or Apple Diagnostics.

Even if the test is negative, you should make a "Genius" appointment at an Apple Store to have the machine tested more thoroughly.

I don't know for sure that the drive is failing. You may (or may not) be able to determine that as follows.

If you have more than one user account, these instructions must be carried out as an administrator.

Triple-click anywhere in the line below on this page to select it:

syslog -k Sender kernel -k Message CSeq "I/O error" | tail | open -f -a TextEdit

Copy the selected text to the Clipboard (command-C).

Launch the Terminal application in any of the following ways:

☞ Enter the first few letters of its name into a Spotlight search. Select it in the results (it should be at the top.)

☞ In the Finder, select Go ▹ Utilities from the menu bar, or press the key combination shift-command-U. The application is in the folder that opens.

☞ Open LaunchPad. Click Utilities, then Terminal in the icon grid.

Paste into the Terminal window (command-V).

The command may take a noticeable amount of time to run. Wait for a new line ending in a dollar sign (“$”) to appear.

A TextEdit window will open with the output of the command. Normally the window will be empty. If you get output like this:

kernel[0] <Debug>: disk0s2: I/O error

the boot drive is failing, or there's some other hardware fault in the storage subsystem.