Mac sticking on loading screen

Hi,

I work in IT and our organisation is about to begin supporting 5 macs on out network (something they have put of for a while)
I'm the new guy so I have been given the task - I got the mac working on the domain but now all of a sudden it won't load a user account.

When I say it won't load an account I mean local or network.
It starts up ok - and brings up the login screen. You click the username from the list and put in the password and as soon as you press login/enter - it freezes.

It shows the white and blue loading bar (which never moves) and then the cursor turns into the rainbow ring. That's it. I can't do anything except restart. I have left it for a fair while and it never goes anywhere.

Any ideas what I could try?
I can't get into the local admin account either through normal or safe mode.

I saw somewhere that I could delete some temp account files - which could cause a problem. Could I boot into a live linux disk and remove them through there?

Don't facy going to the lady who owns the mac to say I broke it so want to try what I can before asking for the disk

I am a Windows user so this is fairly new although I have gathered some experience along the way.
Its a Mac Mini running OSX 10.6...hope thats of some help!

If there are any logs I can get to help find a reason or anything that can be deleted without loosing the data and accounts that would be grand!

Thanks