Question : Problem: What causes HP Slimline to display squiggles on monitor and then totally shutdown?

A client bought an HP Slimline slightly over a year ago and it has performed well until just recently when she reports the monitor begins to display a squiggle pattern or sometimes a broken up pattern and then the machine totally shuts down.  As usual, when I went out to look at it, it never displayed that behavior in two hours of troubleshooting with the computer turned on almost continuously.  The user has Windows Vista Premium Home with all updates installed, Norton Internet Security with all updates installed, and MS Office Home and Student with all updates installed.  I believe SpySweeper is also installed.  The only anomaly I know is that when she was never able to get a cabled connection through the router provided initially by her ISP (now, AT&T), eventually the tech installed a wireless router so she uses a wireless connection of about 2 ft.!  I suggested opening the case and reseating any cards - particularly the graphics card - but beyond that I am not sure what to suggest.   I'd appreciate comments about what might be the cause of the problem and how to diagnose it.

Answer : Problem: What causes HP Slimline to display squiggles on monitor and then totally shutdown?

"squiggle pattern or sometimes a broken up pattern and then the machine totally shuts down" This is usually an indicator of a overheating and/or failing video card. Is the system being stored in a closable cabinet by chance?