If you have the windows cd , try and type in sfc/scannow , this will check your system files for errors, then ask you to put in the windows cd to repair broken files. If this dose not work, maybe your hard drive is failing. I would next try and type chkdsk /c: or whatever drive letter is assigned to the boot drive , for more options type chkdsk /? you will see a list of switches you can use /f will fix errors