17-inch, Mid 2009 mac book pro freezes on startup

Question

My MBP occasionally freezes on startup.
it is a 17-inch, Mid 2009 model details as follows
Processor  2.8 GHz Intel Core 2 Duo
Memory  4 GB 1067 MHz DDR3
Graphics  NVIDIA GeForce 9600M GT 512 MB
Software  Mac OS X Lion 10.7.2 (11C74)
The problem first occured before upgrading to Lion, but does seem to be getting progressively worse.
I start the mbp, get the login screen, enter login and password succefully, and get the opening screen appropriate for my login...
BUT am unable to select anything or perform any actions except using the on/off button to turn the mbp off, when its turned on it usually logs me in succesfully and then behaves as normal until the next login or the one after that ...
Anyone have any ideas what might cause this or if there are any known cures?
Thanks

Answer 1

Reinstalling Lion
Boot to the Recovery HD: Restart the computer and after the chime press and hold down the COMMAND and R keys until the menu screen appears. Alterhatively, restart the computer and after the chime press and hold down the OPTION key until the boot manager screen appears. Select the Recovery HD and click on the downward pointing arrow button.
Repair the Hard Drive and Permissions: Upon startup select Disk Utility from the main menu. Repair the Hard Drive and Permissions as follows.
When the recovery menu appears select Disk Utility. After DU loads select your hard drive entry (mfgr.'s ID and drive size) from the the left side list.  In the DU status area you will see an entry for the S.M.A.R.T. status of the hard drive.  If it does not say "Verified" then the hard drive is failing or failed. (SMART status is not reported on external Firewire or USB drives.) If the drive is "Verified" then select your OS X volume from the list on the left (sub-entry below the drive entry,) click on the First Aid tab, then click on the Repair Disk button. If DU reports any errors that have been fixed, then re-run Repair Disk until no errors are reported. If no errors are reported click on the Repair Permissions button. Wait until the operation completes, then quit DU and return to the main menu.
Reinstall Lion: Select Reinstall Lion and click on the Continue button.
Note: You can also re-download the Lion installer by opening the App Store application. Hold down the OPTION key and click on the Purchases icon in the toolbar. You should now see an active Install button to the right of your Lion purchase entry. There are situations in which this will not work. For example, if you are already booted into the Lion you originally purchased with your Apple ID or if an instance of the Lion installer is located anywhere on your computer.