Lately I have a hard time booting my PC.
It takes about 5 or 6 attempts before the computer finally boots up. I get an Alert Message: Shutdown due to thermal error.
I installed a new video card. Could this be the problem? The video card reqires a 400 watt power supply, the pc is equipped with a 750 watt power supply.
Thanks for any suggestions

Sounds more like your CPU is overheating - Download GPU-Z and CPU-Z they'll tell you how hot each is.

Did you disconnect you CPU fan when you installed the video card?

You can find the utilities below -

CPU-Z
GPU-Z

Post your temps, we'll look up the normal operating range for you.

InlandAZ, thanks for the information.
I downloaded the software yesterday. Unfortunately today I can't even boot the computer. I guess the CPU is permanently damaged.
I did not disconnect the fan during video card installlation.
Is replacing the CPU an option worth looking into? The PC is 4 years old.
My laptop is now 7 years old and still running great.

Did you start getting the shutdown message immediately after you installed the video card? Or was it working for a while after you installed the video card before you started getting the message?

I think most Intel CPU's shut down before they can get damaged from overheating. So it may be that all you need is to change the CPU fan. Or possibly use another connector to attach the fan to the power supply. Sometimes connectors stop working, but most power supplies give you plenty of extra connectors to work with.

What was the video card you replaced, and what did you replace it with.

Did you install updated drivers for the new card?

I changed from a Nvidia GS7900 to a Nvidia 8800GTS. The alert message started about 2 weeks after installation. I had all the drivers already loaded. Did not need the installion cd. I checked all the fans, there are 3. All were working.
Now the pc will not even stay on. It will attempt to boot, I can see the lights and cd/dvdrom light up and then it shuts down within seconds.
I took the pc to a local pc repair shop yesterday. Hopefully they can figure out what's wrong and repair it.

Let us know if they find out what's wrong with it.

Heidi; google "Nvidia 8800 heat problems" & see all the problems other people had with this card.

I have a reply from the pc repair shop and after removing the grahics card, the technician was able to start the computer. Unfortunately, once he shut it off, it took numerous attempts to get it started again and it got worse. The pc was harder and harder to start. He concluded that the motherboard needed to be replaced. He checked the PSU, it checked ok. He also installed a new PSU to double check. So both he and I started looking for a replacement motherboard and after hours and days of searching, we could not locate one for this Dell XPS in the US. There was 1 available in the UK for approx. $ 485 plus shipping cost.
I decided to look for a new system. The shop offered to build a custom gaming computer for me, using some of the parts from the Dell pc, such as the the DVD-RW,the CD-RW/DVD, media card reader,and the hard drive as a 2. HD.

I would think that any motherboard with an ATX form factor would fit the case.

What was the exact model of your PC?

I don't understand that either - any properly sized motherboard should work.

Unless it's a weird-shaped case or something.
Knowing the model number would help.
Then we could look up a picture of it and see if the case is unusual.

The computer is a Dell XPS 710 built in 2007. I also checked with Dell service and parts. They were no help.

I would think a new motherboard would fit in that case without a problem. And if I remember correctly, Inland had a similar model and could tell you for sure.