Done some digging at Adobe, and as you can't link to their pages (have to be a member), I''ve copied pasted some stuff.
Now this is for Acobat Reader 6, but it may work for the free version:
1. Uninstall Reader 6.
2. Use the Export function in the registry editor to backup your registry.
3. Find the following key: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\App Paths\Acrobat.exe
4. Change the text Acrobat.exe to Acrobat.exeold
To do this, rightclick on the Default on the right, and choose Modify.
5. Install Reader 6.
6. Try to view a PDF.
For some reason not everyone has the key mentioned in this procedure. If this is the case for you, substitute the following for steps 3 and 4:
3. Find the following key: HKEY_LOCAL_MACHINE\SOFTWARE\Classes\SOFTWARE\Adobe\Acrobat\Exe
4. Change the text Exe to OldExe
Now, I have the free version on my pc, XP also. I have this key:
So, you may want to try renaming that AcroRd32.exe to AcroRd32.old
Back up the Registry first:
If it doesn't work, just restore that key. You can always do it manually, if need be.