I have Acrobat 5.0 (full version) installed and also Reader X. Acrobat 5 was installed 1st and has been my default. But, I loaded Reader X because Acrobat 5 can't open many newer PDF's. I have gone into Explorer and set Reader X as the "open with" application and double-clicking a PDF in Explorer results in Reader X opening the file. The problem is within various applications which have menu links to PDF's. Many of these still attempt to open the PDF with Acrobat 5. I believe this may be more a Windows XP problem, but thought maybe someone might know what registry item I should look for to correct this problem since it is related to Acrobat.
Adobe doesn't recommend the use of Reader X with anything but Acrobat X. Earlier versions of Acrobat and Reader (any version) will cause problems. Remove Acrobat, re-install Reader X and if needed upgrade to Acrobat X or Remove Reader X and re-install ACrobat 5.