I just installed a router to my desktop for wireless laptop connection. Now everytime I use the desktop it shuts down after about a half hour or so. When I turn it back on says: " Alert! shutdown was due to thermal event! Has anyone ever heard of this? What is it and how do I make it stop?
1) One of the fans in your desktop has stopped working - processor, power supply, or case. I would open the case, power it on, and see if all of the fans are running, and if not get the one that isn't replaced.
2) You have somehow acquired a virus that is playing games with you. I haven't heard of one like this, but that doesn't mean there isn't one.
I really doubt that adding a router to your setup would cause this, unless you have the router directly underneath the desktop. Heat rises, and I presume you have the Actiontec and it does generate a lot of heat.