Lines of colors after entering password in W7 PW field?

This has happened mabe 2-3 times. Boot up computer, get the windows password field, put in pw, next screen is a cross-hatch (vertical and horizontal lines) of various colors. Cursor doesn't work, nothing works, manually shut the computer down. On rebooting, everything is OK??? Nothing appears to be wrong with my monitor (Samsung 193P), or any other functions of computer.

next screen is a cross-hatch (vertical and horizontal lines) of various colors

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This is very likely a problem with the GPU, either it is overheating or defective
Clean the GPU and monitor the temperture using GPUz in safe mode
http://www.techpowerup.com/downloads/1907/TechPowerUp_GPU-Z_v0.4.9.html

I do have an Nvidia card, but don't think that this is an overheating problem as the clored lines appeared in initial morning boot-up, computer that been off all night.