I am having a Windows XP pc here. I had installed the same on Compaq desktop and it looks to be working fine. Here I had tried to use a screen saver and after that there is a blue screen error on the screen. I am getting a constant blue screen message which is STOP: c000021a. I am also gettinga new error logon failed at 0xc0000135 when I try to login .What should I do here to fix the issue.