password does not work idle time lockout occurs

When I boot up the computer my password works just fine. I get into Vista and my desktop loads. However, when the computer locks because of inactivity, the system will not recognize my password. I've also not been able to use the password reset tool for the same reason---it says I have entered an invalid password or username. I have read here that I am not the only one seeing this happening, but have not seen any solution posted yet. Thanks in advance for your help.