There is no Acrobat 64 bit.
Note: Acrobat 8 is unsupported.
You can't download AA8. Do you own AA8? If so, just download the updates. XP 64-bit (and VISTA I think) was supported with AA8.2 or higher. In any case, you need to update. It would help to know what OS you are using. If Win7, you are pretty much on your own. I have yet to hear of someone getting AA8 to work on Win7, 64-bit, only 32-bit success has been reported.