XP Home is freezing

My customer has Windows XP on her computer. What’s happening is that when it gets to the Welcome screen and beyond, it totally freezes. The effect is only temporary, sometimes lasting for a few minutes. I have tried looking for viruses, unusual software, hardware/driver conflicts. I have also booted into Safe Mode; it appears to be okay in that, but I may be wrong. It is very difficult to pinpoint this because it doesn’t happen every time. There are no error messages, and I checked the monitor, no processes taking up the cpu resources. The mouse and keyboard become completely useless. I have never seen this behavior before. I ran a full battery of tests on the cpu, mobo, ram, and hard drive.