That error means that your operating system got deleted, or, at least some of it's core files are gone. You need to reinstall.
Get a linux Live CD.
Use it to backup your data to an external drive: All personnal files, any dowloaded files and your personnal settings...
You now need to reinstall your operating system.
If you can get a copy, I suggest upgrading to Windows 7. Realy superior to vista on all accounts. Faster to.
Reinstall all your programms and restore your datas.
Electro, May 2011
