Start your computer up and as soon as the boot screen appears - tap the 'F8' key a couple of times. this will take you to 'Safe Mode' options screen. in safemode its possible to uninstall any program that may have caused an issue, use system restore to an earlier date when all was ok... or make other configurations such as msconfig (entered into run). the start tab on the window allows users to disable suspect programs from starting up at boot.
pressing 'F8' takes you to the 'Safe Mode' options screen, scroll down using the Up and Down arrow keys and select 'Last good configuration', then press 'Enter'. with any luck, it will boot normally from the last good set-up and all you will have to do is re-install a few recent updates.