My laptop came with Windows 8 and following the dual-boot directions Ubuntu 12.04 was installed and worked perfect, except for completely obliterating Windows 8 and voiding my warranty, meaning neither Toshiba nor the retail center I bought my system from can help me legally
I wouldn't take any claim that installing Linux voids a warranty sitting down. At a minimum, I'd read the warranty myself, and possibly consult an attorney on the matter. On cursory inspection, the 1-year Toshiba warranty here does not cover software, but also does not explicitly say that use of third-party software voids any other aspect of the warranty. I am most definitely not an attorney, but I do know that there are laws that prohibit warranties from being voided for using third-party products (such as third-party oil filters in cars), and it's entirely possible that such a law would apply to third-party software in computers. Of course, you might not want to sue Toshiba over the matter, but if they're denying a warranty claim, the mere threat of a lawsuit might cause them to cave in. Furthermore, I've heard many tales of retail clerks who make ridiculous and erroneous claims to the effect that installing Software X will void a computer's warranty. When called on this, the clerks are invariably proven wrong by manufacturers' agents who actually know what they're talking about.
nor can I return it to get a non-UEFI based system.
These days, there are few or no non-EFI systems being sold, so even if you return the computer, you'll have to exchange it for another EFI-based system.
The problem you're experiencing can almost certainly be resolved without resorting to the warranty; however, it's unclear precisely what your problem is. If Windows was truly overwritten, you'll need to obtain a Windows recovery disc -- at least, assuming you want Windows installed on the computer. You can probably buy such a disc from Toshiba. (Manufacturers have gotten extraordinarily cheap; they no longer ship recovery media with their computers.) Alternatively, see here for options on obtaining legal copies online.
It's entirely possible that Windows has not been overwritten, though; it could be that it's just a question of the Windows boot loader being overwritten or not being launched from GRUB. In this case, you might need to run a Windows recovery disc to restore the boot loader, or simply reconfigure GRUB.
If you don't care about Windows and just want to get Ubuntu up and running, there are numerous questions on this site that cover this topic, such as this one and this one. There are also lots of pages outside of this site that cover this topic, such as this one that I wrote.
In any event, more information is needed before more specific advice can be given. I recommend you run the Boot Info Script from Linux. (Use a live CD, if necessary.) This will produce a file called
RESULTS.txt. Post it to a pastebin site and post the URL that it spits out here. That will give us more details about your setup.