Pre built mod not working

[Max Patti]

Hi there I recently got my hands on an acer aspire tc-780 and it works great but I wanted to game so I found a gtx 550 for sale. I bought the card and plugged it in but I realized the psu didnt have the connector so I bought a molex to 6 pin then wattage on the psu was low so then I bought a evga 500w psu I plugged everything in and there was no response nothing would work if I took the gpu out it would show an error message then continue to start up and then when I started up there was red dots everywhere but I could still see what it was showing I don’t know what to do and I need it to get working soon if someone could help that would be awesome