Computer freezes diagonal pattern?

My families HP Pavillion desktop computer was running slow so i
reformated it to factory settings. Shortly after installing the router
and wireless router antenna the screen looked like this:

now every time I start the computer about one minute after logging on
the screen will "freeze" in a diagonal pattern. It looks like the
pattern is taken from part of the screen just before it froze. i can
open the disk tray when its in this state. the only way to get past it
is to hold the power button and restart the computer. sometimes i get
about 3 minutes, and sometimes the problem happens mid-log in. The
computer is running 32 bit vista. If i boot into safe mode it doesn't
happen. The computer is at least 5 years old. Will reformating it again
possibly fix this? I fear it will freeze during the reformat and
permanently screw the computer up. Do you have any ideas that might fix
this problem? Thanks.

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Typical of a failing or overheated video card.

Reformatting (wrong term, use reinstall the OS) shouldn't fix this.

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I agree with the other answer.
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I agree with the other answer

It could be just a bad fan but the last one I pulled from a machine doing this had bulging capacitors. It was a goner.

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