Windows Installer not working. Help!

I've recently upgraded from a Dell Dimension 4600 to an Inspiron 580s with Windows 7.  Everything is great, all except for the fact that I keep getting this error message anytime I try to download new software.  I can't download itunes either.  Yikes!

Here's the error:  "The windows installer service could not be accessed.  This can occur if the windows installer is not correctly installed.  Contact your support personnel for assistance."

Any assistance would be greatly appreciated.

Thanks!