Is the message coming from Itunes itself? If it is, rather than double clicking the icon to start it, right click on it and choose "Run as Administrator".
There are so many possible causes for this: how many accounts on your PC; corruption in the security settings; and so on. You can work your way through the folders resetting the security by going to Windows Explorer and selecting the folder (not the library if you're using Windows 7) and right click. Choose properties, and then the security tab. It's a bit of a faff to say the least.
You could also try doing a system restore to the last time Itunes worked normally.
Oh and don't buy a Mac. I used to be a fan until they sold out, installed Intel chips and allowed them to run Windows. How pointless was that?