I had this problem on a fresh installed XP machine. I can not login normally through welcome screen. Whenever I clicked on a user, the computer started to freeze and nothing is working after. I eventually solved my problem after an intensive search and try and errors.
1) Try to see if you can logon in Safe Mode with networking (press F8 during booting). It helps you to eliminate areas you do not need to investigate, such as EtherNet card and driver if you can browse the Internet.
2) Video card often causes this problem, disable the display adapter once in Safe Mode and see if you can login mormally. If success, it is the video card conflicts with other system devices (usually it is the CPU to AGP bridge). May need to update display adapter driver to the latest.
3) Look into logfiles in c:\windows\logfiles\watchdogs folder. It may tell you what device having problem. You may see description like "The driver for the display device got stuck in an infinite loop". You know the display adapter has problem.
4) Switch to Classic log on. To do this, go to Control panel, then User Accounts, then Change the way uers log on or off, Un-check the "Use Welcome Screen" checkbox.
In my case, I can log on using classic screen with display adapter enabled, but computer still freezes when running some applications.
5) Adjust motherboard BIO settings, try to disable or turn off anything that is not absolutely necessary, try to do one or two adjustments at a time, otherwise, it will be all mixed up. There are a lots adjustments there, I will not list each one. I still had problem after all of the BIO tweaks.
6) I finally solve computer freeze problem by lower the display hardware acceleration setting. To do this, right click on desktop, then Properties, click on Settings tab, then click on Advance button, then click on Troubleshoot tab, move the Hardware Acceleration slider to the left a little, also clear "Enable Write Combining" checkbox.
Now, for my computer, not only the display adapter enabled, but also runnig in 3D mode without updating the driver (there is no any new driver for it).
Your problem may not be the same as mine, I just share my experience with you, hope it can provide some help to you.
If you computer just recently having problem, it could be the newly installed programs, try to do system restore to earlier save point or do windows setup under window (in Safe Mode if can not logon using either welcome screen or classic login).
Good luck.
