Psexec and safe mode for the win!
PSEXEC allows you to operate via the system context. Even when logged on as a user, even with Admin rights, you do no have full access.
The alternative is to F8 at start and choose safe mode with a command prompt.
The last option is to ensure it's not listed in your boot.ini, config or system properties. I've had this before, a similar situation, whereby Windows chucked a system folder onto a spare 500Gb drive. I couldn't delete it. I had to format the drive.
Yours is a slightly different situation and should be dooable with one of the methods above