I just ran Symantec Anti-Virus and it's flagging literally every since HTML, PHP, ASP/ASPX file on my computer as a virus. That's nearly 600 files on my laptop.
Can this be? How can a text file (html/php/etc..) be a virus.
First: Don't assume its wrong until you have actually checked**** some of those files and confirmed they are false positives. It is possible that some malware compromised your machine and is trying to use HTML files as a vector to the next infection.
The next step will depend on what you find.
 Ways you could check if the files are false positives or not:
Look at the source in your favorite text editor ... is there anything bad added?
If HTML isn't your strong point copy/paste the following into a file and save as something.html:
The above is completely non-malicious but uses a couple different HTML elements that could be triggering a false positive. Now scan this new file and see if it is also flagged as bad. If it is flagged as bad, edit it again and check that the file has not been changed (malware could have changed it). If the file is not changed, but symantec still thinks its bad, then there is something wrong with your anti-virus.