My Mac is really slow to start up / won't boot to desktop / won't start at all

Your user /Library has bad prefences content. You may want to startup without them. Hold the power button in for 5 seconds to terminate operation of the MBP. Let go the power up button and press to startup, immediately pressing the shift key. This will boot into safe mode.

Once the Mac has booted you need to recall what you installed / modified in the session before the problem. Go to the preferences folder in the User Library or the Library and delete those prefs.

If no startup was achieved you may need to boot with the DVD of your current system. Insert the disc and hold the ALT key down on restarting the Mac and select the disc icon when it appears.

Once bootup from the dvd is successfull you need to repair the driver permissionsand repair the drive volume from the DISK UTILITIES menu. This menu is obtained by proceeding with the first screen the dvd presents and then selecting disk utilities from the upper main screen menu.

As far as the problem concerning the slowness or the non boot problem a disk repair or permissions repair or both may get you going again or may repair the speed performance of the Mac, but bigger repairs will need to be performed by a technician.

We're adding questions all the time so watch this space!