We haven't loaded any new hardware or software for several months.
What about Windows updates? AV updates? Firewall updates? Application updates?
it hangs with the response that some software installation is incomplete
Sounds like it could be a failed update.
Again, struggling to understand why you can't manipulate the BIOS configuration. Mr O asks a pertinent question about whether the keyboard is USB / PS2 - can you use the keyboard at all during POST? If so, are you sure you should be pressing enter rather than space / tab / arrow etc?
If you can boot successfully into Safe Mode, the probability is that the problem lies with a device driver or service that would otherwise be loaded.
A significant culprit for boot-up problems is installing hardware drivers via Windows Update - a real no-no IMHO, especially video drivers.
In addition to LKG (which you say you have tried), have you tried System Restore (available in Safe Mode, assuming you have had SR running)?
As others have said, get your data off as the priority, then sort the hardware / software problem.
I would try to get the system booting from CD as the first step, to be able to run Windows Install Repair (not recovery), i.e. select install, then repair existing installation. That is non-destructive and maintains your existing user accounts and settings. OK, you'll have to re-apply all SPs and patches since the CD installation media date (hint - slipstream SP3!).
SD