I always recommend running chkdsk as a first option whenever there is a boot problem.
If you can get into Safe Mode right click on the C drive in Explorer and go Properties > Tools > Check Now (under Error Checking). Check both boxes then click "Start Now". A message will pop up saying that Error Checking will run after you restart the computer. Restart the computer and Error Checking will run automatically after the restart. After itís finished it will restart into Windows automatically.How to start Windows in Safe Mode
If you can't get into Safe Mode insert your Windows XP
CD into the CD drive, and then restart the computer. Click to select any options that are required to start the computer from the CD drive if you are prompted. When the "Welcome to Setup" screen appears, press R to start the Recovery Console
. If you have a dual-boot or multiple-boot computer, select the installation that you must access from the Recovery Console. When you are prompted, type the Administrator password. If the administrator password is blank, just press ENTER.
Type: chkdsk /r
It's important to have a space before the "/".
To exit the Recovery Console and restart the computer, type exit
at the command prompt, and then press ENTER.