First, it is normal for the number of files to change. The System constantly creates temporary files.
(for example: 539.967 files --> 540.345 ...)
That seems quite normal.
You don't have a virus. Read this general introduction to malware and Macs.
http://www.reedcorner.net/guides/macvirus/
Most people here have had very bad experiences with Norton. I don't know about the latest version, which you may have, but I wouldn't allow it on my computer. If you want any A-V (but generally not necessary) either get ClamXav or Sophos free for Mac (some have reported issues with that, however.)
For the sudden loss of 50GB, read this article from Pondini, which includes most of the troubleshooting steps to find missing drive space.
http://pondini.org/OSX/DiskSpace.html
You should also get OmniDiskSweeper (free) and set it to run as root, so you can see any other users.
http://www.omnigroup.com/products/omnidisksweeper/
To run as root, copy/paste in Terminal, hit return, give your pword, which you won't see as you type it in, hit return again.
sudo /Applications/OmniDiskSweeper.app/Contents/MacOS/OmniDiskSweeper
