Q: have formatted my PC (windows) and it now tells me i cant authorise this PC as i'm maxed out?How can i get around this

Deauthorize all then authorize the computer

How can u deauthorize all computers if you cant access any of the ones previously authorized to sign in and deauthorize all?

In iTunes, select iTunes Store, then under "MUSIC QUICK LINKS" on the right hand side of the screen click on Account.  Sign in using your Apple ID, if prompted to do so, and you should see a Deauthorize All button:

Click on this, and then re-authorize any computers than you're currently using with iTunes and your Apple ID. See Deauthorize your computer using iTunes - Apple Support for more details.  Note that you can only use the Deauthorize All function once every twelve months - though you can request that Apple do so on your behalf by contacting Apple Support.