I have an old Optiplex790. I can't power up the desktop and continue to get an amber blinking light and diagnostic codes 1 & 3. I really don't want to spend any money having this fixed; I just want to be able to back up the hard drive. Can I just take out the hard drive and back it up somehow? I apologize in advance if that's a stupid question, but I'm not particularly tech-savvy! TIA
Sure. You can plug it into another working computer, or you could buy a USB to SATA converter cable and copy it onto another drive in a good machine.
Be sure both machines are powered down and unplugged when you change the drive.
You can remove the HDD and using an adapter or drive enclosure can back up the data to another system's drive. I would use a program such as Macrium Reflect Free. It is very user friendly and has great online support documents and tutorials. You can get Macrium here. An enclosure can be seen here but most big box stores have them as well. Just make sure it is a 3.5" enclosure assuming you have a standard desktop HDD. Having said that there are also adapters that are compatible with 2.5" and 3.5" drives such as here. Or docking stations such as here. And I have probably given you way more than you needed to know.
Happy to help out!