Has my computer been hijacked?

I use my personal PC for work, and was called today by the Security dept who informed me that my Windows 8 PC is hosting porn, and people are trying to access it from the Internet.  They said my computer must have been hacked, and that I need to take it off the network a.s.a.p.  They said it's also possible that personal information on the PC is being hacked.   Because it's not a work computer, they would not provide any help or information, only that I should have it looked at, and not to plug it back into the work network.  I use it on my home network as well.  I have Norton 360 and it updates and runs live update and scans daily and has never reported any problems.   I just downloaded and ran "Norton Power Erase" and it doesn't detect any issues either.  All is supposedly okay.  I am really worried, since I have no idea what I am looking for, and if any of my files are infected, so even if I backed it all up, wiped and reformatted the drives, and reinstalled the OS and all the programs, that once I restored my backup files, I could just be reinfecting the PC.  Any suggestions?  I am desperate.

I have noticed lately that my computer is very slow, and that websites often time out before I get to them.  Not sure if this is related or not.