SP2...windows not genuine

Hi
My laptop (acer) automatically installed Windows SP2
it then re-started and now says i have to re install Vista (Vista was already pre-installed, hence no OS disc) as it is now not genuine and when i click to register it again it comes up with a different "error" message"
I decided to Un-install the service pack I dont think it un-installed properly as when doing so it now runs very slow and my desktop is now black and i just see the icons.

Any help with restoring it before the automatic update will be awsome
Thanks

Read this:Error message when you use Windows Vista: "An unauthorized change was made to your license"
Especially check disk any externall good tool Bootable Hard Drive Diagnostics - Vox