i have an old iMac and cannot launch new OSX software updates

We need to narrow down "Old iMac." Please do "About this Mac" from your Apple menu and tell us what the resulting window reports for your Processor type, as in this image:

Please post exactly what your iMac reports at the second red arrow in my example.

If it is a PPC iMac, Leopard 10.5.8 is as far as it an go.

I would second that and the machine will slow down greatly if you go beyond Tiger 10.4