Simple fix: uninstall Microsoft's Insecure Essentials and download and install
avast! Antivirus.
How much did the hard drive replacement cost you, and what is the drive's capacity? If the replacement was a 500GB drive then it should have cost $100 at the very most; if the drive was a terabyte then the replacement should have cost a maximum of $150. If the replacement was more expensive than either of these then I suspect they replaced the motherboard as well because like auto repair shops, tech repair shops simply LOVE to replace more than what is needed so they can make more money with outrageous price tags.
For example, when the graphics card in my Dell Inspiron 530 decided to burn out April 2010 I tried installing various new cards which resulted in my machine not even being able to go into POST, so I took it into Geek Squad at Best Buy and they wanted to backup all of my data, wipe my drives, reinstall Windows and do who knows what else for two-hundred and fifty dollars! Obviously, I was like, "Screw that!" and I did it by myself for free and it didn't even fix my problem! Do you see why I REFUSE to allow tech support companies attempt to diagnose and troubleshoot and fix my problems with my computers?!
Also, I have an HP laptop that no longer powers on and Geek Squad honestly believes it is a motherboard issue! Well let me tell you something; Hewlett-Packard and Dell both ship absolutely HORRIFYING batteries with their laptops. Their circuitry is just...UGH.
I'm going off into a bit of a rant here, but here's the point I am desperately trying to get across: never under ANY circumstances allow tech support companies to reel you into their deathly grip and probe your wallet for every last dollar you possess, because that is just what they do.