1. You can try to press F8 when booting, and selecting "Last
known good configuration" to see if it boots
2. If canot boot, try to put in a windows 7 CD, and then launch
start up recovery.
Press F8 and select Safe Mode with networking. In the event that
safe mode is inaccessible (It should), launch startup recovery or
insert a Windows 7 install disk and select "Repair your
computer"