<p>I want to detect whether adobe reader is installed using VB6. Also if detected that it's not installed, what would be the best solution?</p>

There are crude ways (checking for files on Program files directory), but I'd recommend you declare full-registry functions (not getSetting from vb) as in
http://www.windowsdevcenter.com/pub/a/windows/2004/06/15/VB_Registry_Keys.html
and fetch
HKEY_CLASSES_ROOT\.pdf
If that's there, something capable of reading pdfs is there (which is what you want, right?).
As a bonus,
HKEY_CLASSES_ROOT\.pdf\OpenWithList
has a list (wow) of registered applications that open .pdf files... the key names on that list are programs you can invoke from vb using
shell("start "+ OpenAppName)