Administrator Getting Access Denied Along With CHkdsk Errors At Startup

I have not seen the problem but according to phone conversations when she turns the PC on she gets a black screen with a message that the file system is being checked, but the checking stops with an "unknown error", the black screen flashes and then she gets to her normal log-in screen. This has been going on for a few weeks (ARRRGGHH). The only reason she called me was that she lost internet connectivity this morning.I had her login, open a command prompt, and type in CHKDSK /f but the system returns an ACCESS denied and says she need elevated credentials to run this. As I said she is the only user and she is listed as an administrator under account controls. I had her create a new user account with administrator privileges but NO JOY she still cannot run CHKDSK /f.This is sounding a bit like a virus but I was wondering if anyone had some thoughts on why, or how, administrator privileges are being overridden?