@AL49 let me guess, the command they wanted you to enter was eventvwr (or eventvwr.msc) right? All that does it to open up the built-in Windows event viewer which is a sort of log of the 'things' that windows is doing, and usually there will be loads of events which look for all the world as if they are errors but are really just a normal part of how windows works.
What would have happened is the caller would have got you to open the event viewer and describe what you see, naturally you would see some scary looking entries and the caller would have given you the usual BS spiel along the lines of 'oh my God, this is much worse than we thought, each of those 'errors' is a hacking attempt and very soon the hackers will get in and steal all of your money' - which is quite ironic since that's exactly what the person on the other end of the line is and that's what they are trying to do!
The next step is for them to say 'Don't worry - we can fix this, just download and install this 'special tool' from a particular website' That allows them to gain remote access to the machine (you are perfectly safe up to this point by the way) and they'll go through the motions of apparently checking your machine while in the background they are looking for bank details and installing malicious software to really steal information and send it back to them. The actual remote access software itself is perfectly legitimate - I use them all the time as part of my job!
Eventually they 'find and eliminate' all the problems on your PC, and then possibly even ask for a payment for their helpful services!
It's all laughably obvious, but only if you happen to be technical and can see through all the rubbish they spout - unfortunately many people are taken in by this, certainly enough to make the scam worthwhile.