Acrobat 8 and 9 were out before Windows 7. Because Acrobat 8 was nearing the end of its life, Acrobat 9 was updated to be compatible with Windows 7 but Acrobat 8 was not.
Some users have been able to use some features of Acrobat 8 in Windows 7 32 bit versions. How long these users will have a usable product is not known as any update to Windows 7 could render it unusable.
It is best to upgrade to Acrobat X.