I think the first thing I would do is to go back the the manufacturer. Since it is still in warranty, they should at least try to help get the system working. Maybe they would send you a new hard drive that would be clean.
If they can't help, it seems only 2 other things could be wrong. One would be your system is being corrupted by possibly bad memory or hard drive.
If it isn't that then a virus would seem to be a good bet. There have been some folks that have discovered Trojans, which some anti-viruses will find, or rootkits that probably need special cleaning.
It might help to confirm if you actually do still have admin rights. Go to the start menu-accessories, command prompt and right click and select "Run as Admin"...