I've deleted Google Play Store. How can I reinstall it?

I've been having an issue with my apps where an error "Package file is invalid " keeps appearing. I read a tutorial somewhere which told me to "uninstall updates" for the Google Play Store application. I was told doing this would uninstall the application, and then the device would automatically reinstall Google Play Store. I did this, restarted my phone, but the application never reappeared. I think tried downloading an .apk file to install this manually, but on install the application wouldn't load. Has anyone done this before? How can I reinstall this application? I believe I need it for all future downloads and updates to my applications. For the record, my phone is **not** rooted.