When the splash screen shows, Windows is using NTOSKRNL.EXE to do (basically) the following things:
- initialize the kernel
- start Boot-Start drivers, then System-Start drivers
- creates Session Manager process (Smss.exe) - the first user-mode process
At this point NTLDR should have enabled it's stuff (like the ability to press F8 to get at the Safe Mode screen)
Most often these problems are due to faulty drivers or corrupt registry hives.
The first step, IMO, is to check the hard drive with a bootable diagnostic utility available from the manufacturer of the hard drive.
The next step would be to run chkdsk /r from the Recovery Console. Since you don't have the system disks, you can download an ISO of the recovery console files here:
http://www.thecomputerparamedic.com/files/rc.isoIf those tests pass, then I'd think it was time to do a repair install of XP - and to do that you'll need to have a copy of the installation CD