Try bootting the computer in safe mode.Try logging in all the user a/c's u see in safe mode.If you see the start menu and icons in safe mode than:
start-run-msconfig.
click on startup tab-click on disable all
click on services tab-put a check mark in hide all microsoft services-click on disable all.
click on apply,ok and restart the computer and you should see a normal desktop.
If in safe mode to the same problem persists than:
open the task manager by pressing alt-ctrl-del
click on file-new-nusrmgr.cpl and click on ok.this will give you the option to create a new user a/c.try creating one and see what happens.
OR, from the task manager open cmd and than from cmd-sfc /scannow.
If this doesnt work either use the win xp cd to do a repair installation where you wont loose any files.If you want assistanc in doing repair installation visit :
http://www.microsoft.com/windowsxp/using/setup/support/nostart.mspx