Unfortunately I am now a bit rusty on XP but the next time you do get into Safe Mode try the 'Last known good' option and see if that does anything.
Do you have access to another computer ? It would be worth running the 'Seagate tools for DOS' or your hard drive. Go to the Seagate web-site and download 'Tools for DOS' and burn it to a CD. I thought it downloaded as an ISO file which needs expanding before you burn it to CD but the last time I suggested it to someone I was told it just burnt straight to a CD. So, it may be an ISO file or it may not ! I keep a copy on my desk but it is a long time since I downloaded it.
When you have the CD, power up the computer, open the CD drive, put the disk in and re-start the computer. The Tools for DOS disk is a self-booter so it will fire up your computer and let you check your hard drive. There are two tests on the disk, the long and the short. Run the short first. If that shows any problems run the long one which is much more thorough but takes quite a while longer. This will tell you if there are any serious problems with your hard drive.