Why my windows 7 won’t start in safe mode?

From your question, the exact reason behind your PC's problem is not exactly clear. There seems to be three different possibilities in there. I will discuss all of them below.

1. Graphic Card problem

There is a good chance that your graphic card is not working properly. Seeing red artifacts on the screen is a solid indication for that. If your motherboard has a onboard graphic chip, then you should plug your monitor there to check for an output. In case you don't have that, if possible try restarting your computer with a different graphic card. If switching to a different graphic card solves the problem then it is obviously an issue with your current card.

2. RAM Problem

There are also chances for your Random Access Memory to be corrupted. It is a very common problem these days. You should pull out the memory modules from their slots on the motherboard and check if there are any signs of physical damages, like burning signs or broken parts. If everything is fine, just plug the module in another slot and check if your PC is turning on.

3. Display Driver Problem

Although unlikely in your case, there might be a chance that your display driver is outdated and may cause the screen artifacts. Try turning on your PC in safe mode, if possible and then download the latest available drivers for your graphic card. Install the driver in safe mode and restart in normal mode to check if things are back to normal.