It sounds like your logon utility has become corrupt. While it could probably be fixed manually, it's easier to repair the installation. To repair Windows XP without erasing any of your personal data or programs (although some tightly-integrated programs, such as McAfee AV, may have to be reinstalled to restore full functionality), check out these detailed directions by Microsoft MVP Michael Stevens.
Just to be on the safe side, back up any important files from your corrupt installation first, as this process is not fail-safe. Afterwards, make sure you redownload all Microsoft security updates.
Hope this helps,
I'm running windows XP with SP2 installed - I've kept up to date with all updates and have up to date virus definitions and check routinely for spyware/adware. Unfortunately, my primary hard disk on my computer (a WD 120 gig only 5 months old) contains a version of winXP that now only loads the splash screen then reboots itself in a never ending loop. Luckily I have a second drive with functioning version of winXP that I'm using now, so I'm pretty sure that it's not a hardware issue with any of the peripheral hardware (i've also switched the drive into another comp. and the same thing happens). I can also access all the files on the problematic drive via this good drive. Does anyone have any suggestions on how to use the recovery console to fix winXP without having to resort to a formatting? I've seen people discuss "fixmbr" but am not sure what other options there are. If anyone has a step by step explanation that would be great! many thanks.