OS won't fully load. Goes blank screen with APPCRASH in explorer.exe and ntdll.dll

Wonder if the videocard is toast?

Can you try using another used videocard just to test it?

Another thing you can do is check the Windows Event Viewer to see if there is anything listed under here as a critical, error, or warning( see attached print screen).

Check also to see if there are any .DMP file under the C:\Windows folder.
eventviewer.jpg

can you boot in safe mode, or last known good?
otherwise you may try a repair : http://www.bleepingcomputer.com/tutorials/tutorial148.html        repair Vista

Nothing critical under event viewer. System disc cannot fix it. Nothing in the website tutorial fixes it. It seems to boot up until just before the desktop, then it goes blank white or blank black on the other user's login, with a cursor showing and then ctrl-alt-delete lets you close it out to reboot. It boots into safe mode OK. The problem is there is no recovery partition on the hdd, even though Dell's website says this model has one and  I don't have the OEM disc to re-install Windows.

try uninstall the video driver in safe mode
or update all the drivers
what system is it?

It's Vista Home Premium 64-bit. Tried updating and removal & re-installation, but still no fix.

i asked what system it is - not the OS

Sorry about that! It's Dell Studio 540 desktop.

what drivers did you update?
here they are :  http://www.dell.com/support/home/us/en/19/product-support/product/studio-slim/drivers

you can also try a system restore to a date it was ok in safe mode

There are no system restore points and now cannot download drivers in safe mode either, but thanks for your suggestions

try downloading them on a working pc then
then install

Thanks for that. Did that, but still no better.

i suggest running diags then, at least RAM and disk - i use the ubcd for it:
Hardware diagnostic CD    UBCD

go to the download page, scroll down to the mirror section, and  click on a mirror to start the download
Download the UBCD and make the cd   <<==on a WORKING PC, and boot the problem PC from it
Here 2 links, one to the general site, and a direct link to the download

since the downloaded file is an ISO file, eg ubcd527.iso - so you need to use an ISO burning tool
if you don't have that software, install cdburnerXP : http://cdburnerxp.se/

If you want also the Ram tested - run memtest86+ at least 1 full pass,  - you should have NO errors!
 
For disk Diagnostics run the disk diag for your disk brand (eg seagate diag for seagate drive)  from the HDD section -  long or advanced diag !  (runs at least for30 minutes)

http://www.ultimatebootcd.com/                        ultimate boot cd
http://www.ultimatebootcd.com/download.html             download page

Thanks for staying with this one! HDD & memory both check fine with UBCD530. Full system scan also shows no errors.

Thanks for your efforts. Much appreciated.