It could also just be a pop up window advertisement. I get those even in Vista. Some popular websites do that. When you go to their website it brings up a pop up that makes it look like there is something wrong with your computer, but it is just a sneaky way for them to advertise. If you click on it, it will take you to where you can buy their software. Either just close them and don't worry about it, or download a pop-up blocker for your browser.
You can also download CCleaner - Home
to scan your registry for problems and fix them. It's free software. Maybe that will ease your mind. It's likely to find a lot of problems, but most of the problems these things find are not anything that's a big deal.