[Matryx]

My example is the windows file system, any process can change both a file's read/write access without user intervention.
and Yes, I agree, it seems the Windows series has been designed for hacking in mind.

Not wishing to get into the gory details of it, but programs can write protect/unprotect files without the user's knowledge, and as a result, viruses which change program's code can do exactly the same.
P.S. If you're interested, I run both Win98, Win2K and Good Ole Dos6.2
It is certainly possible in all 3 of these. Linux is thankfully more secure, but not much