I didn't guess it either, which actually got me more mad when I found it out.
I have an old machine which for awhile worked perfectly on a 17" CRT, but I wanted to get rid of it for desk space. I went out and bought a 19" LCD, and hooked it up to this older machine. The machine has an onboard video adapter, with 32MB of shared memory. This put me in a pickle, because I knew I hadn't had problems with the adapter before, but I didn't know if the LCD just needed more muscle to run. So, I put the monitor on another machine to test, and it was fine.
The next day, I got another video card, PCI, to bring home and try out. I put that video card in, and it gave me more problems, but on the Windows side. After I danced around that for a little, I was able to get into Windows. Not only did the problem not go away, it almost got worse. Frustrated, and needing to get work done, I hooked the monitor up to another machine and RDC'ed to this older machine. On the new machine with the 19" hooked up, it still did the same thing.
Grrr...
So, I went back to CompUSA, and they gave me a new one, no questions asked. I was a little hesitant, because the deal was so good on the monitor (at least IMO), that I feared it was being discontinued and would be out of stock. But, I got another, and as I said, it works great
Bad habit I have sometimes is that I skip the simple stuff, and take complicated approaches to everything, oh well.