In your Start menu, right-click "Computer" (or "My Computer" if using XP) and select "Manage".
Expand "Services & Applications".
Open "Services". (Perhaps maximise the screen to better see what's going on.)
Right-click the Bonjour Service and select "Properties".
In the General tab, set the Startup type to "Automatic":
... and click OK.
Restart the PC. Do you still get the message about the service being disabled?
Uninstalling and then reinstalling your iTunes should also get your Bonjour reinstalled.
For the reinstall, use an installer downloaded from the Apple website:
... or you could get it back via your Apple Software Update for Windows.
If you'd prefer not to uninstall iTunes, try the following procedure.
Download and save a copy of the iTunesSetup.exe (or iTunes64setup.exe) installer file to your hard drive:
Download and install the free trial version of WinRAR:
Right-click the iTunesSetup.exe (or iTunes64setup.exe), and select "Extract to iTunesSetup" (or "Extract to iTunes64Setup"). WinRAR will expand the contents of the file into a folder called "iTunesSetup" (or "iTunes64Setup").
Go into the folder and doubleclick the Bonjour.msi (or Bonjour64.msi) to do a standalone Bonjour install.