Had roughly same problem a while ago, just try this:
when you power up insert your Windows master CD or OEM CD, whichever you have been given with the PC.
Very likely you need to power up again; at startup keep pressing the key to enter Bios Mode, this is usually "DEL" but look at bottom of screen, this should be briefly shown as soon as you power up.
Once in BIOS select Bootup Options or Sequence (name can vary from one PC to another) then scroll and select Master or Primary boot to read CD. Validate, usually with F10 and exit, PC should now start rebooting and at one point ask you to press any key to bootup from CD, DON'T DO IT! Just let it go and with any luck your PC should enter normal Windows bootup mode but using CD instead of hard disk. Once you are in you can undo your updates or restore to an earlier date.
When done don't forget to reboot, go back in BIOS and change Primary boot back to Hard Disk.
If it doen't work you can press any key to boot from CD and then attempt a Windows repair, don't have procedure on me right now but if required I can post you a link or if you have access to another machine just do a Google search for "Windows Repair".
Previous suggestions about Graphics card, dust etc also valid, worth a try but don't forget to earth yourself before.
Hope it helps