Help anybody.

I have a Geforce 8600GT and every time I try to watch a movie or play a game I get horizontal lines in a block formation appearing on the screen in windowed or full screen mode. Is my graphics card dying or could there perhaps be a software issue causing this?

Hmm, i'd start with making sure the card is set well in the motherboard. Then, the drivers - do you have the correct ones? 32/64 bit os? You may want to reinstall them. Are there auxiliary power connectors in the video card? Are they connected? Is the card overheating? I think the current drivers have a utility for checking the temp on the card, if not - you should be able to download it somewhere from nvidia.

more testing can narrow down the issue:
does it work with another monitor?
Another cable?
Possible to try in another system?

Let us know how it goes...