If you have recently installed Windows 7 then it might not be your registry. The registry of Windows tend to become a mess after you install and use many programs and then deinstall them again. Not all program properly clean up the registry when they deinstall. When you've recently installed Windows 7 and you haven't installed / deinstalled many programs yet then I'm pretty sure the registry is not the reason.
In your case it might be a malfunctioning program or malware that is causing your system to be unstable. Slow speed and freezing sounds like a program is using a lot of memory causing your computer to be unresponsive (or very slow in responding). Crashes can also be related to that but it might also be that the program you're using is rather badly programmed.