If you upgraded from Vista to Win7, the upgrade can't be uninstalled. Your only option is to re-install Vista (or use a manufacturer's system restore). All of your installed applications will be gone. Make sure to back up all data. Be particularly careful if you have any local email archives.
If Win7 is running badly, something is seriously messed up with your installation. Win7 usually runs as fast or faster than Vista, and Win7 apparently can run OK on systems with weaker hardware. (I've seen it run OK on an old notebook PC with a slow CPU and 1GB of RAM.)
That's why people recommend a clean install rather than an upgrade. Upgrading over Vista probably works OK in most cases, but there may be exceptions. You appear to be one of...