Power the computer on.
keep tapping the F8 key on the keyboard till your met with a menu choice.
You have three options if windows 7.
Use last known good config.
Repair computer.
Or load windows in safe mode.
If it loads n safe mode you have two options
The first is to right click on your C: drive.
Select properties.
Click the tools tab, click error checking, check now.
Click start. then restart the system.
Your second option in windows.
Click on windows start, or the start pearl.
In the search box type: system restore.
Click on search result system restore program.
Now use a restore check point of a date before started to go wrong about six weeks before if there is one.
Hi light it.
Click restore windows to this point.
Warning any work you did or anything like a program you installed will be removed.
You will loose any work or programs that were used or installed after the date of the restore point.
But should get the comp working and booting back into windows without looping.
Try the other steps first see if it fixes your problem. system restore should be your last option if all else fails.