My mac book pro will not fully boot up. I have the grey background with the apple sign and the timer going round...but 20 minutes later still no desktop

You could try doing a hard shut down (holding the power button or yanking the battery) and then doing a safe boot by holding the shift key at startup:

http://support.apple.com/kb/HT1455

Also, not sure if this applies in your case, but I've had that happen when there's a USB drive inserted either in the computer or a connected external keyboard. I did a hard shut down, removed the drive and it went through a normal start up.

Look at this support article for possible solutions;

http://support.apple.com/kb/TS2570

Ciao.

You may also read this user tip article;

https://discussions.apple.com/docs/DOC-3353

Ciao.