Just a question, (and forgive me if I missed it in your other posts) do you have an actual store bought full version xp? Or do you have an OEM or pre installed XP? There are a few repair options here and no need to jump into a re=install right away.
The keys may vary, but typically tap the F8 or F9,F10 ,F11 ,F12 or F1 or F2. This is a varient on different machines but typically tap the key and you may come to some repair options. F1 , F2 typically bring you into Cmos setup which you don’t want. So F8 -F12 will bring you some options and F8 is typically safe mode which you probably know. If you can bring up the options screen, choose last known good configuration. Since you already tried safe mode, is there a repair\non destructive recovery option? You may try tapping R at boot to get into some recovery option and do a non destructive repair. Once again, if OEM you may have a recovery partition, or disk\s or can order them.
If not OEM…(Full)
You can do a fake install and go to repair. If you boot from the disk and go into setup as if you were installing, you will see and option to repair the installation. Here is one of many links…
There are other ways without doing a complete reinstall, especially with a full version. If you can in fact get into safe mode, you should have done a system restore for a first step. If you fear you will lose anything, back up your information (which you should do anyway) and then attempt the repair.
( Many times these steps will get your pc back to running order nicely.) (:KWL)
I have to ask, did you possibly interrupt the update process or run other programs? Doing this can cause major grief from any Anti-V or firewall. I just fixed my son's last night as he interrupted (couldn't wait for update) messed around a bit and his Anti-v wouldn't update any more, said it needed to be registered, when the only registration is when you put the disk in with the key, had to restart with disk and repair as well. Updates should be taken more seriously when being put on a pc but many don't know this, like me, when I run my security updates, I put it off until I can stop any outright running programs and this seems to work best. <This is just FYI and I am in no way saying you did this, I have gotten thrashed upon for even mentioning some may have made a mistake , lolll, so I want to make this clear that it's NOT an accusation. ;) >
Just some info if you feel you are stuck…