Reboot, start tapping F8 until you get a black screen. In this we are using things before the GUI(Graphical User Interface) ie: Windows loads so we have better chance of fixing things.
If this problem just started select Last Known Good Configuration
I have recovered many using this on XP...
command prompt
complete pc restoredellfactoryrestore
graphical user interface
last known good configuration
memory diagnostic
repair options
startup repair
system restore
windows 7
