itunes/iphone troubleshooting!

is it possible to update my default computer to sync my iPhone 4 with?? Got a new computer(PC) and can't get my phone to sync or let me put any new music from it on there. I've tried clicking the "Manually manage" whatever else it says and then click cancel when it asks if i want to erase and sync, just like the apple help page says to do, but that doesn't work. once i click the cancel button it unchecks the manually manage button too. never had that problem before, but still want to know if i can update my default computer for syncing purposes!