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We can try some basic troubleshooting steps and see if that improves the speed of your Mac. Reset PRAM and NVRAM (Solution 1)
Restart in Safe mode (Solution 2) Make sure that the computer is shut down. Press the power button. Immediately after you hear the startup tone, press and hold the Shift key. Release the Shift key when you see the gray Apple and progress indicator. Let the Mac boot up... Run Disk Utility in Single User Mode(Solution 3) Turn your Mac off. Turn it back on and this time hold down the Command (Apple) key and the S keys at the same time. This will boot your Mac into Single User Mode. Then type the following command... /sbin/fsck -fy (note the space between fsck and -fy) If you can't run /sbin/fsck -fy then type only fsk - fy Hit return and the disk utility should run and let it finish. When it finishes and there is no error, then type reboot and hit return. Let the Mac boot up. (Disk Utility will fix all system errors, bad preferences files etc)
Also, you can download and install this 10.6.8 Update from here : http://support.apple.com/kb/DL1399
Please let me know if that didn't work, so we can continue troubleshooting. Don't Rate my answer until you are satisfied.