My laptop just keep on popping up the message:
C:/Users/"My name"/ appdata/local/temp/~tmpa.exe
The NTVDM CPU has encountered an illegal instruction. It then gives a line of number with last saying: choose "close" to terminate the application.

I has a 2 buttons, one saying "close" another saying "ignore".

It happened about 3 times now, I press close. What do I do? Is it dangerous?

You've got a malware infestation. Not good at all.

Google "tmpa.exe" and you will see a wide list or ways to "remove" it.

I had something like this once, however I was not successful at ridding myself of it and had to wipe the hard drive to the computer and start over.

Be ready to take it to a computer shop. Because if you have this bug, chances are you've got countless other spywares and trojans on your computer.

Everything else is fine. The only difference is the message pops up every once in 15 minutes

Most spyware is designed to be discreet.