If might have some virus or hard drive has problems. Or just something messed up windows. You can't really know which is the real cause. What you want to do here is to get your data. I can help you try get them without getting in windows at all.

If you don't want any data, yes just reinstall windows will be must.

If you can use another computer go here http://www.hirensbootcd.org/download/

and download Hiren's BootCD 15.2.zip.

Open that zip and get the Hiren's BootCD 15.2.ISO out of there. The ISO is a CD in electronic form.

You need to open this ISO using Isoburner software or use the included to burn it on a blank CD.

Then open your broken computer and throw this new made CD in and boot from it.

You can go here to learn how to do this.

http://answers.yahoo.com/question/index?qid=200611...

Then you will see a menu. Choose mini Windows XP.

You will see Windows XP loading on your screen, will take some time and then you will see a desktop with tools to use.

There you can play around using Windows Explorer to find your hard drive stuff and copy them on another external usb drive.

After that you are safe to re install windows or worst case scenario to replace your hard disk.

Good Luck!