Before the need for a repair install as outlined in the well written article found at
http://www.michaelstevenstech.com/XPrepairinstall.htm you may want to give the system file checker a run. If you have a full install disk of Windows onhand you would put that into the cd or dvd drive used and exit the installation screen when that comes up.
The system file checker is started by typing "sfc /scannow" at either the Run prompt off of the Start menu or the command prompt found in accessories and press the enter key or "ok" button to see that run. This will first start verifying system files as part of the process and would ordinarily prompt for the Windows cd if any files are found damaged or missing. At that time those found damaged or missing are repaired/replaced as needed.
There is an easy way to have Windows load directly to the desktop without the need for the login prompt if you are running a sole user machine. A section in the older Windows Registry Guide known as Winguide 2003 is free to review and download at
http://www.winguides.com/software/download.php?id=1
To review the section on auto logging into Windows go to
http://www.winguides.com/registry/display.php/13/ This manual edit of the system registry bypasses the problem if done correctly as described there by going straight to the Windows desktop and removes the constant logon prompt.
Don't rule out malware completely while it still may not sound likely.
There are different forms of viruses, adbots, spybots, and other malwares that can less obvious at first until problems are seen. the links here provide free online tools for scanning your system for "bugs" as well as security checks. Symantec has the two of those right off at
http://security.symantec.com/sscv6/...d=22&pkj=NCGSCKMRKRFPECDMEYI&setjsax=1&bhcp=1
A pair of other sites scan for trojan downloaders that can get on your system and often remain hidden while downloading other garbage.
http://www.windowsecurity.com/trojanscan/ is one while Ewido known owned by Grisoft creator of AVG has one at
http://www.ewido.net/en/download/ On the link there look on the left side for the man sitting with a laptop and the "scan now" button is right under that. Besides ""bugs" from a virus or spyware you could have had a bad install of a program now seeing problems when Windows starts. But give these methods a try before using the repair install method.
