The amount of overscan you have isn't really that bad at all, for a TV. I usually get more overscan than that with the Sonys I've used.
TVs have a certain amount of overscan built into them, maybe 3-5% of the image or even more. This is because you won't normally be able to tell/care about losing the edges of a typical TV image, and otherwise people would be constantly whinging about black bars filling up unused screen areas (they feel like they are missing something, poor souls). Adjusting overscan through the service menu is generally the best way to minimise it - often you need an original remote to access service mode.
You can also look at editing your video modes with ArcadeOSD to increase your porch settings and make the modes a better fit for your TV. However, you will only get a limited amount of flexibility here as the modes will only take so many extra porch lines before going out of sync.
You can also try running games in resolutions that are slightly larger than the games normally use to account for the overscan, eg use 336x248 for 320x240 games, 288x240 for 256x224 games etc. If using Groovymame You may have to force the issue by specifying individual game resolutions with <romname>.ini files. Note that doing this may cause vertical refresh rate related issues if GM is having too much trouble matching them for your chosen resolution.
If overscan still bugs you in Windows, try increasing the width of your taskbar (click/drag/pull) to make it easier to click on icons, or even move it to the side or top! Don't use full-screen windows. Alternatively, try running Windows desktop in a larger resolution such as 800x600, if your TV is able to do so.