I can most likely conclude that you're running the 32-Bit Vista. Retailers like Best Buy well almost never offer 64-Bit versions that are pre-installed and purchasing from a OEM's website usually gets you the 32-Bit by default unless you specifically choose the 64-Bit. Heck, some don't even offer it as an option

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Go into your BIOS and move your disc drive up in the list (Commonly contained under a "Boot Sequence" section or option) so that it is the first device the computer tries to boot from. When you get to the page where it gives you the link to "install Windows" after you have booted from the disc, look at the lower-left hand corner of the screen and click on "Repair your computer". It is important to know that the recovery iso you downloaded is NOT a full Vista disc, and well not re-install Windows for you, but only repair an existing installation. Clicking on "Install Now" well give you nothing but errors. It should then look for exisitng installations. If it can find a problem right away, it'll prompt you to repair and restart the computer. Otherwise, it'll bring you to a screen where you can choose from all of the avaiable Vista installations it can find. If you can get to this screen and don't see at least one item, then you'll either need to re-install Windows or get a new hard drive as your current one may be failing. From there, you are presented with a recovery options menu, which should allow you to perform actions such as restoring the computer to a previous restore point or using start up repair to repair common misconfigurations that prevent Windows from starting.
Some other options:
1) See if you can boot from a recovery partiion and restore the system to its factory default settings. Some systems contain one and are accessed using a key combination at start up before Windows begins to load. You'll need to check the user manual that came with your computer or other documentation from Gateway to tell you what the key combination is, as each brand may be different.
2) Contact Gateway if the computer is covered under warranty