How long has this been occurring, have you just installed new software or drivers that perhaps aren't programmed well enough to use Fast User Switching? See if any of these help.
294427 - Windows XP Stops Responding at the Welcome Screen
816873 - Windows XP Welcome Screen Appears to Stop Responding (Hang) During Logon
Some things for you to look at.
Logon - Logoff loop, also caused by BlazeFind
Log in Logs straight back Out
Thread779-924408
logs you on loads personal settings then logs you off?
Thread779-919265
A second reason can be security policy settings:
A User Logon Request Is Rejected Without Any Messages
"The security log on this system is full" message when you try to log on to a computer that is running Windows XP or Windows Server 2003
HOW TO: Install and Use the Recovery Console for Windows XP (Q307654)
Try running ChkDsk /r from the Recovery Console.
An easy to follow recovery console description when unable to start computer due to corrupt registry.
This is a layman's version of Q307545 in simple language.
Removing adware & spyware
FAQ608-4650
HOW TO: Verify Unsigned Device Drivers in Windows XP
How To Use the Fast User Switching Feature in Windows XP
Error Message: Your System Has No Paging File, or the Paging File Is Too Small