Microsoft provides this really handy tool called "Windows 7 Upgrade Advisor" which scans your PC and lets you know if your system is capable of running Windows 7 and explains any potential compatibility issues. In general, if your PC/laptop had Windows Vista, it can run Windows 7 and from our experience, most computers handle Windows 7 better than Vista. But if you were sticking to Windows XP and don't have one of those "Vista Ready" branded computers, this utility will come very handy. Just download, install and launch the application. It will scan your computer and any external device connected to your computer like printers, scanners etc. for compatibility with Windows 7 and generate a detailed report with potential issues and suggestions to resolve them. For example, it might let you know that you need an updated driver for your Graphics Card and can direct you as to where to get it.
connect all the external devices that you generally use with your computer (external hard-drives, printers, scanners, other input devices, Tv-tuner cards, projector etc) while running this utility so that it checks their compatibility as well.