Installing Snow Leopard Problems *Please Help*

Hi I've installed Snow Leopard but now my mac won't start up. (it's a early 2008 mac pro so it should be able to handle it)

All that happens is it loads past the apple logo, the rainbow wheel pops up, the background loads and the search icon (magnifing glass) appears but nothing else at all beyond that. It just waits for ages (a few times that note has appeared that say's I need to restart the computer).

I've tried...
1) re installing it
2) reinstalling it on a different partition (that had windows on before)
but nothing seems to work.

I'm not really sure what to do. I really need to keep all my data so I want to be careful.

Any help greatfully appreciated.

 

I think I've managed to sort it by installing another clean copy of it on a partition of my drive. (4th time lucky)

I still have no idea why there were such problems though.