Do you have the XP CD?

When the machine is booting up it should give you an option to press a key to enter the BIOS (normally get a message at the bottom to tell you what key to press to 'Enter Setup'.

Go into the BIOS and in the Boot option set CD as first boot device. Press F10 to 'Save & Exit'.

As soon as the machine closes to restart put your CD in.

When the machine has booted from the CD press the R key to launch the Recovery Console. Select the XP installation you want to repair and press Enter. It will ask you for the Administrator password (if it has one) and then give you a command screen.

Type these one at a time pressing Enter after each command.

D: [ENTER]
CD I386 [ENTER]
EXPAND USERINIT.EX_ C:\WINDOWS\SYSTEM32 [ENTER]

After the last command has run you should see a message similar to "1 file(s) copied".

Type Exit and press Enter. Your machine should restart. (Remove the CD as soon as you hit Enter).

Go back into the BIOS and reset the first boot device as HDD. Save & Exit.

Hopefully the logon issue is now resolved.