SL 10.6.2 won't boot

[powerless.rm]

MacBookPro freezes in grey screen at start up, showing apple and windmill.

Last week it froze in blue screen. After trying everything except wiping the disc (it wouldn't reinstall over 10.6.2, I kept getting a crash window), I tried Safe-start. That failed, but at the next attempt to start it normally it suddenly got past blue screen and booted.

Now I'm not getting past grey screen. I've once again tried everything short of wiping the harddrive – which I don't want to do since I've got some new stuff there that's not been backed up yet.
So far:
C-start to SL system disk
Disk Utility/repair permissions + repair disk
Apple Hardware test (No problems)
Reset SMC/ reset PRAM
Safe start

I assume that 10.6 won't install over 10.6 without first erasing everything? But will that work?

Any suggestions or explanations? Can I back up content through the SL OS disk, an external hard drive, before wiping the my harddrive and trying re-install?

BTW, I had installed SL over Tiger on this old MBP using the inexpensive OS disc I bought at an authorized Apple dealer, using their advice.

I'd be very grateful for help. I'm at my wits' end and feel like I have to buy a new laptop, but am reluctant to get another Mac.

 

[JediMeister]

Reinstalling 10.6 will always do what was known in 10.4 and 10.5 as an archive and install, preserving users and network settings. Unless you actually go into Disk Utility and format the drive, recreating the partition table if you would like, it will not wipe any of the information on the computer. If you are concerned about even the slightest chance of data loss you should use Target Disk Mode to access your important files and copy them from your affected Mac to another.

 

[powerless.rm]

Thanks for replying. You confirm what I'd pretty much gathered so far from forum postings. I figure the hard disc is shot, after 4 years. I didn't know that could happen as I'd never kept a laptop this long. Guess Mac is better than most after all.
But is it still weird that no problem is reported when using Apple hardware test or with Disk Utility's "repair disk" function? I've even run fsck a few times until I got the mesage that everything seems OK.
Still won't boot.