Hi rikstaparr,
Try booting into the BIOS and see if you have the same display problems there. This will help us determine if it is truly a hardware problem or perhaps a software or driver problem of some sort (or at least narrow down the possible options - depending on the results). If it works in BIOS, then try booting into Safe Mode and see if it works there. If so, then there's a good chance it isn't a hardware problem per se but something else causing the problem. If not, then we will still have learned some valuable information.
If not, try booting to a bootable disk like the genuine Vista Installation Disk or a repair disk you can make (Y
ou can make a bootable Recovery Disk using http://neosmart.net/blog/2008/windows-vista-recovery-disc-download/ along with burning software like: http://www.snapfiles.com/get/active-isoburner.html and, of course, a blank CD (perhaps made on a different computer or perhaps in safe mode with networking if that is working). To boot to the CD you may need to change the BIOS to make the CD-drive first in the boot sequence. To do that, wait for the screen that tells you the F key to push to access the boot menu or boot setup. Push it quickly. Make the changes, save your work, and exit. Put the CD in the drive and reboot. When prompted, push any key to boot from the CD.) and see if that makes any difference (as by booting to the CD you will not be using Vista or some of the other drivers or hardware components). If this works, then chances are good it isn't a hardware problem per se but something to do with Vista or what's active when Vista installs. If not, then chances of a hardware problem are increased.
If it has an external monitor adapter or connection (or you can get one rather inexpensively), then try attaching an external monitor to the computer and see if the view is any better. If so, the problem may in fact be the monitor and neither the MOBO or graphics card. However, it could still be either of those two since the connections for the external monitor are different from those of the internal monitor - but it could be an important step in diagnosing the problem.
Adding another graphics card does not seem to be an option. From the specifications
Model Content Page, I could not tell if the graphics card is a part of the motherboard or can be removed and replaced. I suppose you either need to contact Toshiba to find out or check the card and motherboard to see if it is in a removable slot or permanently affixed. If removable, one option would be to purchase a replacement (with a guarantee that you can return it for a full refund if you don't like it - not just a replacement, but a full return and refund) that works with your system and version of Vista and exchange them and see if that resolves the problem. If so, the issue is the graphics card and you've just fixed the problem and all is well. If not, then chances are excellent it is not the graphics card, so just replace the original and return the new one for a refund - and you will have essentially eliminated one of the possible causes of the problem. Doing this with the monitor screen or the MOBO won't be quite so easy as you'll need to get them from Toshiba and their return policy may not be as accommodating (to say nothing of the fact that those replacements will be considerably more difficult to install to say the least).
I know of no diagnostic tests for your MOBO or monitor specifically (other than exchanging them with known good ones).
I assume you have tried various personalization settings (themes, colors and appearance, themes, resolutions, DPI settings and everything else related) and that has had no effect. I've checked your Toshiba model and there are no available diagnostic utilities that can be downloaded from the website.
Between these options and those in the link recommended by Richard, let's see where that leaves us and what else, if anything, we can do.
I hope this helps.
Good luck!
.