My system has adobe 9.1 but when i want to open some pdf it still says version 8 or 9 required

It sounds like you may have more than one version of Adobe Reader and/or Adobe Acrobat installed on your computer (not recommended) and the system is probably confused about which one to use when opening a .pdf document.
Right-click any .pdf file located on your system, and then, click Open With and then click Choose Default Program.


In the window that opens, select the program that you want your system to use when you open a .pdf file (in your case it should be Adobe Reader 9.1). It *should* be listed under the Recommended Programs section but if it is not then you will need to use the Browse button in order to located Adobe Reader on your system. If the correct program is already selected, then I suggest you unselect it and then reselect it. Make sure the "Always use the selected program to open this kind of file" check-box is checked. Then hit the OK button.
As stated earlier, it is not recommended (and not supported) for you to have more than one version of Adobe Reader installed at a time. You should do the following if the above solution does not work for you:
In Windows, go to Start Button > Control Panel > Programs > Uninstall a program
Look through the list of your installed programs for Adobe Acrobat and/or Adobe Reader. Uninstall them both (if you have more than one installed) then download and run the Microsoft Installer Cleanup Utility to remove all traces of Acrobat Reader.
Now re-install your latest version of Acrobat Reader only.