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I am completely at my wits end with this. I have had the computer for quite some time now and everything was running well until I come home, turn it on one day and BSOD before logon screen. It says something about nv4_disp.dll which I believe is a nvidia driver. I have downloaded the new driver, but I can't even get into my computer in safe mode without the same thing happening. Please help.

First, if you are content to do this and have no backup of your files, fit another hard drive to the system and reinstall windows onto that. You can then reconnect your old hard drive and pull any data files across.
Otherwise, boot the system from the windows install CD and follow the wizard. Ignore the first offer of repair and proceed until you get to the second offer; select and the routine will replace critical windows files and hopefully fix the problem.
This does not always work, but is worth a try.
The cause is most likely to be a failing hard drive so until you have found out what caused the error, don't trust it.

I have attempted to reinstall/repair Windows on the old hard drive and to install it on a new one but I am having the problem that when the Windows disk loads and displays the choices any keyboard I have connected to the system won't work for some reason. I have tried both USB and PS2 and so suspect that something more sinister is behind this lack of function.

If you have no keyboard access at boot with a PS/2 then providing you have tried more than one PS/2 keyboard I would suspect the motherboard. To use the USB keyboard you need to enable the legacy USB devices in the BIOS. As you say, there may be something more sinister. You could try booting with alterating each one of your memory modules (if you have more than one) just in case this is a failed memory module.