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,
John