Pink screen

Clarifications

Could be an issue with the administrator account's user profile. I've seen similar things happen when the user portion of the registry gets corrupt.
C:\Users\%username%\NTUSER.DAT
In your case C:\Users\Administrator\NTUSER.DAT

The tricky part to this scenario is that the guest account is the only one available for login, therefore the easiest way to do this will probably be to run a system restore.

What I would recommend is attempting a system restore back to the last time everything was working.

You will need to launch this as administrator. If UAC does not prompt you for the administrator user and password (password may be blank). Then you can hold shift on the keyboard right click on system restore and say runas administrator.

System Restore quick guide for Vista
http://windows.microsoft.com/en-us/windows-vista/What-is-System-Restore

Hope this helps...

some things... for one the admin account in vista is turned off by default... So my guess is that whoever built this thing named the first account admin... either that or enabled the admin account and blew away the user account, for some unknown reason.

but I would suggest jumping into safe mode, after POST, but before windows loading screen, hit F8.... or twards the end of the HP loading screen start tapping F8, You should get a prompt to start with safe mode, and more choices... Use safe mode, log in as the admin and see what you get. if its fine, I would suggest running a virus scan + maleware scan + go to run command (windows key+r) type cmd then type SFC /SCANNOW once thats done run chkdsk /r and reboot... allow the PC to run check disk... then try logging in as you normally would.