If starting windows on safe mode does not work you can try to repair by recovery console. But first you need to have a Windows XP CD, then follow this instructions:
1.Reboot the computer with the Windows XP CD in the optical drive. First you will need to accept the license agreement, press F8 to accept it and proceed.
2.Setup will search for existing windows installations, at this point it should give you the options to install a fresh copy of windows or to repair. In this case press R key to rapair.
3.After you press the R key, the recovery console should start up. It should show a message like ''Microsoft Windows XP Recovery Console'' etc etc. and command prompt. It will ask you wich Windows XP installation you would like to log onto, you should type 1 and press enter, then it will ask you for the administrator password, type it and press enter.
Now, don't really know what is your problem exactly, so lets try this:
4. Type ''fixmbr'' and press enter, this fix the master boot record.
Type ''copy d:\i386\ntldr'' and press enter.
Type ''copy d:\i386\ntdetect.com
5. Now lets rebuild the boot.ini, in case that it's gone or corrupted.
Type ''bootcfg /rebuild''.
Recovery console will try to locate all installed copies of windows and ask if you want to add them to the new BOOT.INI file that it's going to create, chose yes for the ones you want.
6.That's it. Type exit, and take out the CD from the optical drive.
If that does not work try checking the hard drive. Do this by entering recovery console as before. As soon as you get to the command prompt type ''chkdsk'' this runs the check disk utility, and will tell you if your disk is good or damaged. If it's damaged you will need to replace it.