getting "HTTP Error 503. The service is unavailable"

i have in my network SBS 2008 with Sharepoint 3.0
The former System Administrator installed Sharepoint 2010 whitout migrating or uninstalling version 3.0 which caused some of the services of the SP not to function correctly.
I tried to uninstall the SP 2010 but it did nothing so i used "Installer Cleaner" and remove it.
after restarting the server the company web site is available and i get error 503
when i try to reinstall the sp 3.0 which located under "C:\Program Files\Windows Small Business Server\Bin\CMPNENTS\WSS\setup.exe" - it close the application by saying:
"an error occured during installation, and setup was unable to complete"

Any suggtetion how solve this issue?

Thank you in advance
Sagi

what i had to do is start the services in application pool section on the iis