The Windows login screen will load the same winlogon.exe, either winlogon.exe has been patched by malware or one of the related dll's is infected, corrupt or the wrong version.
Its possible you have an infection causing this.
Follow the order given below to properly disinfect your PC
1.) Make a boot AV disc then boot from the disc and scan the hard drive, remove any infections it finds, I prefer the Kaspersky disc myself. The New 2010 Kaspersky disc can update the AV dat files if you are connected to the internet at the time of scan and is suggested to update before the scan.
http://www.techmixer.com/free-bootable-antivirus-rescue-cds-download-list/
2.) Then:
Install free MBAM, run the program and go to the Update tab and update it, then go to the Scanner Tab and do a quick scan, select and remove anything it finds.
http://download.cnet.com/Malwarebytes-Anti-Malware/3000-8022_4-10804572.html
3.) When MBAM is done install SAS free version, run a quick scan, remove what it automatically selects.
http://www.superantispyware.com/download.html
These last 2 are not AV softwares like Norton, they are on demand scanners that only scan for nasties when you run the program and will not interfere with your installed AV, these can be run once a day or week to ensure you are not infected. Be sure you update them before each daily-weekly scan.
