Which particular files are slow to open? A file needs an "application", so it's not really the file, but the application which is probably taking time to open. Check your hard disk space, see if it's getting close to capacity. In the task manager, check the memory usage of the running applications. When the "slow" file opens in it's application, check that application's memory usage. And when all else fails, format and re-install.
If Windows complains about missing disk space on your media, it's time for a clean installation. A clean install will remove all traces of Windows. So a clean installation is the most effective way to get rid of all traces of Windows, right? If that's a little too hard, just delete your installations of Windows, then reinstall from scratch. And then remove everything. Check on your hard drives and software. There might be errors or missing files. Try looking through the program's uninstall file. What's so great about this clean installation method? Well, it installs without modifying system files or registry settings, and there's no need to reinstall. After a clean installation, you can't reinstall any new software. Windows will automatically detect your disk space and automatically make a suitable installation. It will not install files which are already on your hard drives, and won't install existing programs which are.