In this response I am making two assumptions:
First off, when you say "my vista screen is smaller than my monitor" along with "[in excel] I can't see my sheet tabs at the bottom", it sounds like the image being displayed on the monitor is actually bigger than the screen itself, i.e. there is graphic information that seems to stretch beyond the physics limits of the monitor itself. Or, perhaps it is vice-versa, all of the information is on the screen, it's just that the information isn't "filling" the screen edge to edge, so to speak.
Second, I am assuming that it is NOT the case that you have recently bought and installed a new monitor, or recently installed a new windows operating system, such as upgrading to vista from xp, or complete wipeout and reinstall, etc. If this is the case, then see further below.
At any rate, the simplest answer to all this is to simply change the resolution settings of your monitor. In vista, these settings can be changes by going to the Start menu, followed by selecting "Control Panel", then clicking on "Classic View" (only do this if the word "classic view" is not already in the bolder of the two fonts, with a white dot next to it -- if the is a white dot next to this, then don't click Classic View) - - - then click "Personalization", and finally "display settings".
In the "Display settings" window, there should be a slider bar that is movable with the mouse -- this sliding bar controls the screen resolution. By changing this setting you are essentially changing how big vista thinks the screen is. You may have to do a bit of playing around with this slider bar to find the right setting. Chances are excellent that there is a setting on this slider that will produce a reasonable view of the screen that you can at least live with.
One of the reasons why this might fail is that computer screen manufacturers don't seem to have reached any agreement so far on exactly how big a screen should be. I'm pretty sure that there are odd-ball sizes of screens that vista just seems to have problems with, although this problem is growing less and less severe every year.
If it is the case that you have recently bought a new monitor, or recently did a re-install or upgrade to the operating system, my first suspicion would be that some kind of crucial video driver either has not been installed, or has not been installed correctly. This situation can be dealt with, but before we get too far into this, let me know if changing the settings in the control panel has any success, or even effect at all.