|"i think my cpu is 64bit"|
You can't think it is, you have to KNOW it is, otherwise you're wasting your time. If you have a P4, it's not 64-bit.
And I agree with Fingers...XP 64-bit should be avoided. It was never a popular OS & never was well supported in terms of drivers or software.
Why do you want to go with a 64-bit OS anyway?