PC Kept Restarting, Now No Input Signal

I have a four year old HP Pavilion D4100Y. Over the past few days it has suddenly restarted several times. I have several computers so I didn't notice the issue at first, but realized that every time I looked at the HP computer it was back to the login screen from a restart.

Now I have a "no input signal" to the monitor and cannot access the PC at all, including through Remote Desktop or Go to My PC.

Here are the troubleshooting steps I have taken so far:

1. Unplugged the monitor and attached another monitor that I know is working. Got the same "no input signal" message.

2. Unplugged everything, shut down, reattached and restarted. Same issue.

3. The PC appears to be on; I see the power button lit and I can open and close the CD tray.

Here is what changed on this computer over the last couple of days:

1. Yesterday I purchased and ran RegCure registry cleaner software.
2. I downloaded the Xvid codec to play a video file.

I've been reading about possible bad video cards, bad motherboards, etc. I haven't done much troubleshooting without the aid of a monitor so I am looking for what to try next.

Not too worried about losing data as I have everything backed up . . .  but would certainly like to resurrect this puppy.

Any next steps are appreciated!