Mac Mini suddently terribly slow
Solution 1:
Take it to the Apple store nearest and talk to one of the Geniuses. They will most likely boot your make in safe mode which displays a bunch of system messages on the screen. They will help you for free.
Solution 2:
If the issues are this widespread, there may be an issue with your hard drive. Make sure that you have a backup of any important data that you can't afford to lose.
You can try to boot from your OS X installation disc or recovery partition and run a check on your hard drive using the Disk Utility.